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EVOLUTION

The Modem Synthesis

EVOLUTION

The Modern Synthesis

JULIAN HUXLEY, m.a., d.sc.. f.k.s.

li'.JDC

LONDON

George Allen & Unwin Ltd

HRST PUBLiSHED IN 1 942
SECOND IMPRESSION 1943
THIRD IMPRESSION 1944
FOURTH IMPRESSION I 945
FIFTH IMPRESSION I 948

This book is copyright under the Berne Convention,
Apart from any fair dealing for the purposes of
private study, research, criticism or review, as
permitted under the Copyright Act, 1911, no
portion may be reproduced by any process tvithout
written permission. Enquiry should he made to the
publisher © George Allen and Unwin Ltd., 1942.

CENTRAL ARCHAEOLOQIO:^
LIMIARY, K|W DELHI.

Ace. No ^

* rt *

PRINTED IN GREAT BRITAIN BY
BRADFORD AND DICKENS
DRAYTON HOUSE, LONDON, W.C.I

Dedicated to T. H. Morgan:
many-sided leader in biology’s advance.

PREFACE

In 1936 , 1 had to find a subject for the presidential address to the
Zoology Section of the British Association. After some hesitation,

I chose “Natural Selection and Evolutionary Progress”, since it
seemed to me that these were two interrelated topics of funda-
mental biological importance, yet on which much misappre-
hension existed. Even among professional zoologists the modem
conception of natural selection and its mode of operation is quite
different from that of Darwin’s day, but much of the research
on which the changed outlook is based is so recent that the new
ideas have not spread far. The idea of evolutionary progress, on
the other hand, has been undeservedly neglected. Thus it seemed
to me valuable to attempt to give a broad account of the two
concepts and their relation to each other.

The result exceeded my expectations. So many of my col-
leagues expressed interest and the wish that the address might
be available in more extended and more permanent form, that
I decided to essay expanding it into a book.

The result is the present volume. I am fully conscious of its
hmitations and imperfections, but I believe that it will serve a
useful purpose. The writing of it has so much clarified my own
thinking , and the discussion of the problems that arose with
colleagues has resulted in so many ideas and points of view which
were novel both to them and to myself, tlut I am encouraged
to beheve it will be of general service. I also feel sure that a
classification and analysis of evolutionary trends and processes as
observed or deduced in nature, and the attempted relation of
them to the findings of genetics and systematics, is of first-class
importance for any unified biological outlook; and since others
better equipped than I seem reluctant to attempt the task, I have
tried my hand at it.

I owe a great deal to J. B. S. Haldane’s The Causes of Evolution',
but though our books overlap, they differ considerably in scope
and treatment. Dobzhansky’s, Waddingtou’s, and Goldschmidt’s
valuable and distinctive books did not appear until much of the
present volume was already in proof; but I have tried to take

8 evolution: THE MO0ESN synthesis

advantage of them where possible. My debt to R. A. Fisher s
work is obvious. Fisher has radically transformed our outlook on
the subject, notably by pointing out how the effect of a mutation
can be altered by new combinations and mutations of other
genes. Any originahty w'hich this book may possess lies partly
in its attempting to generalize this idea still further, by stressing
the fact that a study of the effects of genes during development k
as essential for an understanding of evolution as are the study of
mutation and that of selection. I may also claim that taxonomic
data have not previously been analysed on so large a scale in the
light of modern genetic and evolutionary views. Equally obvious
is my debt to the Morgan school and to Goldschmidt; but clearly
this would apply to any modern book dealing with evolution.

I have taken for granted in my reader an acquaintance with the
basic principles of Mendelian heredity and the major groups of
the a nim al ^gdom. Widi this equipment, the layman interested
in biology will, I hope, find the book suited to his needs, though
I hope that it vrill appeal mainly to professional biologists
interested in the more general aspects of their subjects.

I would like to record my special gratitude to Mr. E. B. Ford,
of Oxford, who has read the book in typescript, and with whom
I have discussed all the genetic problems involved: he has been
fertile in suggestion and prodigal of assistance. To Professor
L. T. Hogben, F.E.S., I owe several valuable suggestions on the
evolution of species. I should also like to thank Professor R. A.
Fisher, f.k.s., Professor H. J. Muller, Dr. C. D. Darlington,,
F.R.S., Professor Hale Carpenter, Dr. W. B. Turrill, and Mr. Moy
Thomas for help and advice; and particularly Mr. James Fisher
for valuable assistance in revising the book for press.

The time is ripe for a rapid advance in our understanding of
evolution. Genetics, developmental physiology, ecology, system-
atics, paleontology, cytology, mathematical analysis, have .all
provided new facts or new tools of research; the need to-day is
for concerted attack and synthesis. If diis book contributes to
such a synthetic point of view, I shall be w:ell content.

THE ZOOLOGICAL SOCIETY, LONDON

March 1942

CONTENTS

PAGE

Preface . , . . . . . 7

Chapter i. The Theory of Natural Selection

I. The theory of natural selection .

Z. The nature of variation . ,

3. The eclipse of Darwinism ....

chapter 2. The Multiformity of Evolution

I. The heterogeneity of evolution

... 29

2. The paleontological data

. 31

3. Evolution in rare and abundant species .

. 32

4. Adaptations and their interpretation

. 34

5. Adaptation and selection .

. 37

6. The three aspects of biological fact

. 40

7. The main types of evolutionary process

. 42

Chapter 3. Mendelism and Evolution

1. Mutation and selection

2. Genes and characters

3. The alteration of genic expression . . .

4. The evolution of dominance . . . . .

5. Types of mutation . ...

6. Special cases: melanism; polymorphism; fluctuating

populations

7. Mutation and evolution . ...

115

chapter 4.. Genetic Systems and Evolution

1. The factors of evolution

2. The early evolution of genetic systems .

3. The meiotic system and its adjustment .

4. The consequences of polyploidy .

5. Species-hybridization and sex-determinatioii :

elusion . . • ...

. 125

. m

. X36

' :I43;

con-

eyoxiition; the modern synthesis

PAGE

Chapter 5. The Species Probkm; Geographical

Speciation

1. The biological reality of species . . . • ^ 5 ^

2. The different modes of speciation; sticcessiona! species . 170

3. Geographical replacement : the mtnrc of subspecies . m

4. Clines (character-gradients) , . - • . 206

5. Spadai and ecological factors in geographical divergence . 227

6. Range-changes subsequent to geogiaphiol differentiation 243

7* The principles of geographical differentiation. . . 2^59

chapter 6. Speciation, Ecological and Genetic

1. Local versus geographical differentiation . . . 2 3

2. Ecological divergence . » • • * •

3. Overlapping species-pairs ... • • • ^^4

4. Biological differentiation . . . . • • ^95

5. Physiological and reproductive differentiation . . 308

6 . Special cases 3 ^^

7. Divergence with low competition; oceanic faunas , . 323

8. Genetic divergence . . . . • • • 328

9. Convergent species^formation . . . . • 339

10. Rericniate differentiation . . * • • . 351

11, illustrative examples .... • • • 336

Chapter j. Speciation, Evolution, and Taxonomy

I* Different types of spedadon and their results . , . 382

2. Spedes-formadon and evoludon . , . . . 387

3 . MckIcs of spedadon and systemadc method . . . 39®

Chapter 8. Adaptation and Selection

1. The omnipresence of adaptadon . . , . . 4^2

2 . Adaptadon and funedon; types and examples of adaptadon 417

3. Reguiarides of adaptadon 430

4. Adaptadon as a relative concept 438

5. Preadaptation , . . . • * » * 449

6. The origin of adaptations; the inadequacy of Lamarckism 457

7. The origin of adaptations: natural selection . . . 466

8. Adaptadon and selection not necessarily beneficial to the

,$pedes '■ '

478

CONTENTS

PAGE'

Chapter 9 . Evolutionary Trends

1. Trends in adaptive radiation , - . . . 486

2. Tile sclecteve determimtioa of adaptive trends . . 494

3. The apparent orthogenesis of adaptive trends . . 497

4. Noil-adaptive trends and orthogenesis .... 504

5. The restriction of variation • : . , . .516

6. Conseqnentia! evolution: "the consequences -of differential

development 525

7. Other consequential evolutionary trends . . . 543

Chapter 10 . Evolutionary Progress

1. Is evolutionary progress a scientific concept ? * . 556

2. The definition of evolutionary progress . . . 559

3. The nature and mechanism of evolutionary progress . 562

4. The past course of evolutionary progress . . . 5^9

5. Progress in the evolutionary future . , . . $ 7 ^

Bibliography 579

Index

Subjects 6x5

Organisms. . . . • • • • ^^3

Authorities . . . • • • • • ^38

CHAPTER 1

THE THEORY OF NATURAL SELECTION

I* Tlie theory of mtura! selectioii ij'

2. The nature of vaiiatioii . . . . ■ . , . . p. 17

3. The ecHpse ofDarwinisin 22

L THE THEORY OF NATURAE SELECTION

Evolutioii may lay claim to be considered the most central and

the most important of the problems of biology. For an attack
upon it we need facts and methods from every branch of the
science— ecology, genetics, paleontology, geographical distri-
bution, embryology, systematics, comparative anatomy — ^not to
mention reinforcements from other disciplines such as geology,
geography, and mathematics.

Biology at the present time is embarking upon a phase of
synthesis after a period in which new disciplines were taken up
in turn and worked out in comparative isolation. Nowhere k this
movement towards unification more likely to be valuable foan
in this many-sided topic of evolution; and already we are seeing
the first-fruits in the re-animation of Darwinkm.

By Darwinism I imply that blend of induction and deduction
wliich Darwin was the first to apply to the study of eyolution.
He was concerned both to establish the fact of evolution, and to
dkcover the mechanism by which it operated; and it was precisely
because he attacked both aspects of the problem simultaneously,
that he was so successful,* On the one hand he amassed enormous
quantities of facts from which inductions concerning the evolu-
tionary process could be drawn; and on the other, starting from
a few general principles, he deduced the further principle of
natural selection.

^ This method is not, as has sometimes been asserted, a circular argument.
See discussion in Huxley, 19386.

evolution: the modeen synthesis

It is as well to remember the strong deduetive element in
Darwinism. Darwin based his theory of natural selection on three
observable facts of nature and two deductions from them. The
first fact is the tendency of all organisms to increase in a geo-
metrical ratio. The tendency of all organisms to increase is due
to the fact that offspring, in the early stages of their existence, are
always more numerous than their parents; this holds good whetlier
reproduction is sexual or asexual, by fission or by buddmg, by
means of seeds, spores, or eggs.* The second fact is that, in spite
of this tendency to progressive increase, the numbers of a given
species actually remain more or less constant.

The first deduction follows. From these two facts he deduced
the struggle for existence. For since more young are produced
dian can survive, there must be competition for survival. In
amplifying his theory, he extended the concept of the struggle
for existence to cover reproduction. The struggle is in point of
fact for survival of the stock; if its survival is aided by greater
fertility, an earlier breeding season, or other reproductive function,
these should be included under the same head.

Darwin’s third fact of nature was variation: all organisms vary
appreciably. And the second and final deduction, which he
deduced from the first deduction and the third fact, was Natural
Selection. Since there is a struggle for existence among indi-
viduals, and since these individuals are not all alike, some of the
variations among them wiU be advantageous in the struggle for
survival, others unfavourable. Consequently, a higher proportion
of individuals with favourable variations will on the average
survive, a higher proportion of those with unfavourable vari- ^
ations will die or fail to reproduce themselves. And since a great
deal of variation is transmitted by heredity, these effects of
differential survival will in large measure accumulate fiom
generation to generation. Thus natural selection will act con-
standy to improve and to maintain the adjustment of animals
and plants to their surroundings and their way of hfc.

A few comments on these points in the light of the historical

* I be only exception, so far as 1 am aware, is to be found in certain human
populations which M far short of replacing themselves.

THE THEORY OF NATURAL SELECTION LJ

development of biology since Darwin’s day will clarify both his
statement of the theory and the modem position m regard to it.

His first fact has remained unquestioned. All organisms possess
the potentiality of geometric increase. We had better perhaps say
increase of geometric type, since the ratio of offspring to parents may
vary considerably from place to place, and from season to season.
In all cases, however, the tendency or potentiality is not merely
to a progressive increase, but to a multiplicative and not to an
additive increase.

Equally unquestioned is his second fact, the general constancy
of numbers of any species. As he himself was careful to point out,
the constancy is only approximate. At any one time, there will
always be some species that are increasing in their numbers,
others that are decreasing. But even when a species is increasing,
the actual increase is never as great as the potential: some young
will fail to survive. Again, with our much greater knowledge
of ecology, we know to-day that many species undergo cycHcal
and often remarkably regular fluctuations, frequently of very
large extent, in their numbers (see p. i lO Elton, 1937)- But this
fact, although it has certain interesting evolutionary consequences,
does not invalidate the general principle.

The first two facts being accepted, the deduction from them
also holds: a struggle for existence, or better, a straggle for
survival, must occur.

The difficulties of the further bases of the theory are greater,
and it is here that the major criticisms have fallen. In the first
place, Darwin assumed that tlie bulk of variations were inherit-
able. He expressly stated that any which were not inheritable
would be irrelevant to the discussion; but he continued in the
assumption that those which are inheritable provide an adequate
reservoir of potential improvement.*

As Haldane (1938, p. 107) has pointed out, the decreased
interest in England in plant-breeding, caused by the repeal of the

* Origin of Species (6th ed., one vol. ed.. p. 9) : “• • • any vmation which is
not inherited is unimportant for us. But the number and diversity of uiheritable
deviations of structure, both those of slight and those of considerable physio-
logicaT importance, arc endless. No breeder doubts how strong is the tendency
to inheritance: that like produces like is his fundamental belief.” And so on.

x6 evolution: THB MODEHN SyNtHESIS

Com Laws, led Darwin to take most of his evidence from
ammal-breeders. This was much more obscure than what the
plant-breeders in France had obtained: in faa Vilmorin, before
Darwin wrote, had fully established the roles of heritable and
non-heritable variation in wheat.

Thus in 'Darwin’s time, and still more in England than m
France, the subject of inheritance was still very obscure. In any
case the basic laws of heredity, or, as we should now say, the
principles of genetics, had not yet emerged. In a full formulation
of the theory of Natural Selection, we should have to add a
further fact and a further deduction. We should begin, as he did,
with the fact of variation, and deduce from it and our previous
deduction of the struggle for existence that there must be a
diffeTPtitial survival of different types of offspring in each genera-
tion. We should then proceed to the fact of inheritance. Some
variation is inherited: and that fraction will be available for trans-
mission to later generations. Thus our final deduction is that the
result will be a differential transmission of inherited variation.
The term Natural Selection is thus seen to have two rather
different meanings. In a broad sense it covers all cases of differ-
ential survival; but from the evolutionary point of view it covers
only the differential transmission of inheritable variations.

Mendelian analysis has revealed the further fact, unsuspected
by Darwm, that recombination of existing genetic units may
both produce and modify new inheritable variations. And this,
as we shall see later, has important evolutionary consequences.

Although both the principle of differential survival and that of
its evolutionary accumulation by Natural Selection were for
Darwin essentially deductions, it is important to realize that, if
true, they are also facts of nature capable of verification by observ-
ation and experiment. And in point of fact differential mortality,
differential survival, and differential multiphcation among
variants of the same species are now known in numerous cases.

The criticism, however, was early made that a great deal oi
the mortality found in nature appeared to be accidental and non-
selective. This would hold for the destruction of the great
majority of eggs and larvae of prolific marine animals, or the

THE THEORY OF NATURAL SELECTION VJ

death of seeds which fell on stony groiaid or other unsuitable
habitats. It remains true that we require .nany more quantitative
experiments on the subject before we can Imow accurately the
extent of non-selective elimination. Even a very large percent^e
of such elimination, however, in no way invalidates the selection
principle from holding for the remaining fraction (see p. 467).
The very fact that it is accidental and non-selective ensures that
the residue shall be a random sample, and will therefore contain
any variation of selective value in the same proportions as the
whole population. It is, I think, fair to say that the fact of differ-
ential survival of different variations is generally accepted,
although it still requires much clarification, especially on the
quantitative side. In other words, natural selection within the
bounds of the single generation is an active factor in biology.

2. THE NATURE OF VARIATION

The really important criticisms have fallen upon Natural Selection
as an evolutionary principle, and have centred round the nature
of inheritable variation.

Darwin, though his views on the subject did not remain
constant, was always inclined to allow some weight to Lamarckian
principles, according to which the efects of use and disuse and
of environmental influences were supposed to be in some degree
inherited. However, later work has steadily reduced the scope
that can be allowed to such agencies: Weismann drew a sharp
distinction between soma and germplasm, between the individual
body which was not concerned in reproduction, and the heredi-
tary constitution contained in the ^rm-cells, which alone was
transmitted in heredity. Purely somatic effects, according to him,
could not be passed on: the sole inheritable variations were
variations in the hereditary constitution.

Although the distinction between soma and germplasm is not
always so sharp as Weismann supposed, and although the principle
of Baldwrin and Lloyd Morgan, usually called Orgamc Selection,
shows how Lamarckism may be simiikted by the feter replace-
ment of adaptive modifications by adaptive mutations, Weis-

i8 evolution; the modern synthesis

mann’s conceptions resulted in a great clarification of the position.

It is owing to him that we to-day classify variations into two
fundamentally distinct categories — modifications and mutations
(together with new arrangements of mutations, or recombina-
tions; see below, p. 20). Modifications are produced by altera-
tions in the environment (including modifications of the interna
environment such as are brought about by use, and dis^),
mutations by alterations in the substance of the heredity
constitution. The distinction may be put in a rather different but
perhaps more illuminating way. Variation is a study of the differ-
ences between organisms. On analysis, these differences ma.y turn
out to be due to differences in environment (as with an etiolated
plant growing in a cellar as against a green one in light; or a sun-
tanned sailor as against a pale slum-dweller) ; or they may turn
out to be due to differences in hereditary constitution (as between
an albino and a green seedling in the same plot, or a negro and
a white man in the same dty); or of course to a simultaneous
difference both in environment and in constitution (as with the
difference in stature between an undernourished pigmy and a
well-nourished negro). Furthermore, only the second arc inherited.
We speak of them as genetic differences: at their first origin
they appear to be due to mutatiors in the hereditary constitution.
The former we call modifications, and arc not inheritable.

The important fact is that only experiment can decide between
the two. Both in nature and in the laboratory, one of two indis-
tinguishable variants may turn out to be due to environment, the
otiiCT to genetic peculiarity. A particular shade of human com-
plexion may be due to genetic constitution making for fair
complexion plus considerable exposure to the sun, or to a genetic-
ally dark complexion plus very little tanning: and similarly for
stature, intelligence, and many other characters.

This leads to a further important conclusion: characters as
such arc not and cannot be inherited. For a character is always the
joint product of a particular genetic composition and a particular
set of environmental circumstances. Some characters are much
more stable in regard to the normal range of environmental
variation than are others — ^for instance, human eye-colour or

THE THEORY OF NATURAL SELECTION

hair-form as against skin-colour or weight. But these too are in
principle similar. Alter the environment of the embryo sufficiently,
and eyeless monsters with markedly changed brain-development
are produced.

In the early days of Mendelian research, phrases such as “in
fowls, the character rose-comb is inherited as a Mendehan
dominant” were current. So long as such phrases are recognized
as mere convenient shorthand, they are harmless; but when they
are taken to imply the actual genetic transmission of the characters,
they are wholly incorrect.

Even as shorthand, they may mislead. To say that rose-comb
is inherited as a dominant, even if we know that we mean the
genetic factor for rose-comb, is likely to lead to what I may call
the one-to-one or biUiard-ball view of genetics. There are assumed
to be a large number of chararters in the organism, each one
represented in a more or less invariable way by a particular factor
or gene, or a combination of a few factors. This crude particulate
view is a mere restatement of the preformation theory of develop-
ment: granted the rose-comb factor, the rosc-comb character,
nice and clear-cut, will always appear. The rosc-comb factor, it is
true, is not regarded as a sub-microscopic replica of the actual
rose-comb, but is taken to represent it by some form of unanalysed
but inevitable correspondence. ,

The fallacy in this view is again revealed by the use of the
difference method. In asserting that rose-comb is a dominant
character, we are merely stating in a too abbreviated form the
results of experiments to determine what constitutes the difference
between fowls with rose-combs and fowls with single combs.
In reahty, what is inherited as a Mendelian dominant is the gene
in the rose-combed stock which differentiates it from the single-
combed stock: we have no right to assert anything more as a
result of our experiments than the existence of such a differential
factor.

Actually, every character is dependent on a very large number
(possibly all) o^ the genes in the hereditary constitution: but
some of these genes exert marked differential effects upon the
visible appearance. Both rose- and single-comb fowls contain all

E¥OItlTION. : THE MODEEN ' S YNTHESIS '

the genes needed to build np a full-sized comb: but Tose^ genes

build it up according to one growth-pattern, “single” genes

according to anotber.

This prindpie is of great importance. For instance, up till very
recendy the chief data in human genetics have been pedigrees of
abnormalities or diseases collected by medical men. And in
these data, medical men have usually been obsessed
with the implications of the ideas of “character-inheritsmce”.
When die character has not appeared in orthodox and classical
Mendelian fashion they have tended to dismiss it with some such
phrase as “inheritance irregular”, whereas further analysis might
have shown a perfecdy normal inheritance of the gene concerned,
but an irregular expression of the character, dependent on the other
genes with which it was assodated and upon differences in
environment (see discussion in Hogben, 1933)-

This leads on to a further and very vital fact, namely, the
existence of a type of genetic process undreamt of until the
Mendelian epocL In Darwin’s day biological inheritance meant
die reappearance of similar characters in offipring and parent, and
impHed the physical transmission of some material basis for the
, chiacters. What would Darwin or any nineteenth-century
biologist say to facts such as the following, which 'flow form part
of any elementary course in genetics ? A black and an albino mouse
are mated. All their offspring are grey, like wild mice: but in the
second generation greys, blacks, and albinos appear in the ratio
9 : 3 : 4 . Or again, fowls with rose-comb and pea-comb mated
together produce nothing but so-called walnut combs: but in the
nert generation, in addition to walnut, rose, and pea, some sit^le
coml» are produced.

To the biologist of the Darwinian period the production of the
grey mice would have been not inheritance, but “reversion” to
the wild type, and the reappearance of the blacks and whites in
the next generation would iuve been “atavism” or “skipping a
generation”. Similarly the appearance of single combs in Ae fowl
CToss would have been described as reversion, while the pro-
duction of walnut combs would have been regarded as some
form of “sport.”

THE THEORY OF NATURAL SELECTION 21

In reality, the results are in both cases immediately explicable
on the assumption of two pairs of genes, each transmitted from
parent to ofispring by the same fundamental genetic mechanism.
The “reversions”, “atavisms”, and “sports” are all due to new
combinations of old genes. Thus, although all the facts are in one
sense phenomena of inheritance, it is legitimate and in some ways
desirable to distinguish those in which the same characters
reappear generation after generation from those in which new
characters are generated. As Haldane has put it, modem genetics
deals not only with inheritance, but with recombination.

Thus the raw material available for evolution by natural selec-
tion falls into two categories — mutation and recombination.
Mutation is the only begetter of intrinsic change in the separate
units of the hereditary comtitution: it alters fhe nature of genes.*

Recombination, on the other hand, thoi^h it may produce
quite new combinations with quite new efiects on characters,
only juggles with existir^ genes. It is, however, almost as impor-
tant for evolution. It cannot occur without sexual reproduction:
and its importance in providing the possibility of speedily com-
bining several favourable mutations doubdess accounts for the
aU-but-universal presence of the sexual process in the life-cycle
or organisms. We shall in later chapters see its importance for
adjustii^ mutations to the needs of the organism.

Darwinism to-day thus stiU contains an element of deduction,
and is none the worse for that as a scientific theory. But the facts
available in relation to it are both more precise andmorenumerous,
with the result that we are able to check our deductions and
to malfp. quantitative prophecies with much greater fullness than
was possible to Darwin. This has been especially notable as
regards the mathematical treatment of the problem, which we
owe to R. A. Fisher,}. B. S. Haldane, Sewall Wright, and others.
We can now take mutation-rates and degrees of advantage of one

* Strictly speaking, this applies only to gene-mutation. Chromosome-muta-
tion, whether it adds or subtracts chromosome-sets, whole chromosomes, or
parts of chromosomes, or inverts sections of chromosomes, merely provides
new quantitative or positional combinations of old genes. However, chromo-
some-mutation may alter the effects of genes. Thus we are covered if we say that
mutation alters cither the qualitative nature or the ct]ectivc action (inoiuding
the mode of transmission) of the hereditary constitiitio.n.

evolution: THE MODERN SYNTHESIS

mutation or combination over another, which are within the
limits actually found in genetic experiments, and can calculate

the rates of evolution which will then occur.

If mutation had a rate that was very high it would neutralize
or over-ride selective eflFects: if one that was very low, it would
not provide sufficient raw material for change ; if it were not
more or less at random in many directions, evolution would nm
in orthogenetic grooves. But mutation being in point of fact
chiefly at random, and the mutation-rate being always moderately
low, we can deduce that die struggle for existence will be
effective in producing differential survival and evolutionary
change.

3. tHe eclipse of DARWINISM

The d^ a tb of Darwinism has been proclaimed not only firom the
pulpit, but from the biological laboratory; but, as in the case of
Mark Twain, the reports seem to have been greatly exaggerated,
since to-day Darwinism is very much alive.

The reaction against Darwinism set in during the nineties of
last century. The younger zoologists of that time were discon-
tented vitith the trends of their science. The m^or school still
seemed to think that the sole aim of zoology was to elucidate the
relationship of the larger groups. Had not Kovalevsky demon-
strated the vertebrate affinities of the sea-squirts, and did not
comparative embryology prove the common ancestry of groups
so unlike as worms and molluscs? Intoxicated witli such earlier
successes of evolutionary phylogeny, they proceeded (like some
Forestry Commission of science) to plant wildernesses of family
trees over the beauty-spots of biology.

A related school, a litde less prone to speculation, concentrated
on the punuit of comparative morphology within groups. This
provides one of the most admirable of intellectual trainings for
the student, and has yielded extremely important results for
science. But if pursued too exclusively for its own sake, it leads,
as Radi has pithily put it in his History of Biological Theories, to
spending one’s time comparing one thing with another without

THE THEORY OF NATURAL SELECTION 23

ever troubling about what either of them really is. bi other words,
zoology, becoming morphological, suftcred divorce from physi-
ology. And finally Darwinism itself grew more and more
tlicorctical. The paper demonstration that such and such a
character was or might be adaptive was regarded by many writers
as sufficient proof that it must owe its origin to Natural Selection.
Evolutionary studies became more and more merely casc--books
of real or supposed adaptations. Late nineteenth-century Darwin-
ism came to resemble the early nineteenth-century school of
Natural Theology, Paley redivivus, one might say, but philo-
s(.>phically upside down, with Natural Selection instead of a
Divine Artificer as the Deus ex machina. There was little contact
of evolutionary speculation with the concrete facts of cytology
and heredity, or with actual experimentation.

A major symptom of revolt was the publication of William
Bateson’s Materials for the Study of Variation in 1894. Bateson had
done valuable work on the embryology of Balanoglossus; but his
sceptical and concrete mind found it distasteful to spend itself oh
speculations on the ancestry of the vertebrates, which was dicn
regarded as the outstanding topic of evolution, and he turned to
a task which, however different it might seem, he rightly regarded
as piercing nearer to the heart of the evolutionary problems.
DcUberatcly he gathered evidence 'of variation which was dis-
continuous, as opposed to the continuous variation postulated by
Darwin and Weismaun. The resultant volume of material, though
its gadicring might fairly be called biassed, was impressive in
quantity and range, and deeply impressed the more active spirite
in biology. It was die first symptom of what we may call the
period of mutation theory, which postulated that large mutations,
.and not small ‘"continuous variations”, were the raw material of
evolution, and actually determined most of its course, selection
being relegated to a wholly subordinate position.

This was first formally promulgated by de Vries (1901, 1905)
as a result of liis work with the evening primroses, Oenothera,
and was later adopted by various other workers, notably T. H.
Morgan, in liis fiT.st gcnctical phase. The views of the early
twentieth-century geneticists, moreover, were coloured by the

24 EVOI.UTION: THE MODERN SYNTHESIS

rediscovery of Mendel's laws by Correns, de Vries, and Tscher-
-mak in the spring of 1900, and the rapid generalization of tliem,
notably by Bateson.

Naturally, the early Mendehans worked with clear-cut differ-
ences of large extent. As it became clearer that niendelian inheri-
tance was universal, it was natural to suppose all niendelian
^ors produced large effects, that therefore mutation was sharp
and discontinuous, and that the continuous variation which is
obviously widespread in luture is not heritable.

Bateson did not hesitate to draw the most devastating conclu-
sions from his reading of the mcndclian facts. In his Presidential
Address to the British Association in 1914. assuming first that
charge in the germplasm is always by large mutation and
secondly that all mutation is loss, from a dominant something to
a recessive nothing, he concluded that the whole of evolution is
merely an unpacking. The hypothetical ancestral amoeba con-
tained— actually and not just potentially— the entire complex of
life’s hereditary factoK. The jettisoning of different portions of
this complex released the potentiaUties of this, that, and the other
group a^ form of life. Selection and adaptation were relegated
to an unconsidered background.

Meanwhile the true-blue Darwinian stream, leaving Weis-
mannism behind, had reached its biometric phase. Tracing its
origin to Galton, biometry blossomed under the guidance of
Karl Pearson and Weldon. Unfortunately this, the first thorough
appheation of mathematics to evolution, though productive of
many important results and leading to still more important
advances in method, was for a considerable time rendered sterile
by its refusal to acknowledge the genetic facts discovered by the
Mendehans. Botli sides, indeed, were to blame. The biometricians
istuck to hypothetical modes of inheritance and genetic variation
on which to exercise their mathematical skill; the Mendelians
refused to acknowledge that continuous variation could be
genetic, or at any rate dependent on genes, or that a mathematical
tlicory of selection could be of any real service to the evolutionary
biologist.

It was in this period, immediately prior to the war, that the

THE THEORY OF NATURAL SELECTION 25

legend of the death of Darwinism acquired currency. The facts
of mendclism appeared to contradict the facts of paleontology,
the theories of the mutationists would not square with the
Weismanrdan views of adaptation, the discoveries of experi-
mental embryology seemed to contradict the classical recapitu-
latory theories of development. Zoologists who clung to
Darwinian views were looked down on by the devotees of the
newer disciplines, whether cytology or genetics, Entwkklmgs-
tnechanik or comparative physiology, as old-fashioned theorizers;
and the theological and philosophical antipathy to Darwin’s
great mechanistic generalization could once more raise its head
without fearing too violent a knock.

But the old-fashioned selectionists were guided hy a sound
instinct. The opposing factions became reconciled as the yoimger
branches of biology achieved a synthesis with each other and
with the classical disciplines: and the reconciliation converged
upon a Darwinian centre.

It speedily became apparent that mendelism applied to the
heredity of all many-celled and many sir^le-ceUed organisms,
both animals and plants. The mcndelian laws received a simple
and general interpretation: they were due in the first place to
inheritance being particulate, and in the second place to the
particles being borne on the chromosomes, whose behaviour
could be observed under the microscope Many apparent excep-
tions to mcndelian rules turned out to be due to aberrations of
chromosome-behaviour. Segregation and recombination, the
fundamental mcndelian facts, arc all but universal, being co-
extensive with sexual reproduction; and mutation, the further
corollary of the particulate theory of heredity, was found to occur
even more widely, in somatic tissues and in parthenogenetic
and sexuaUy-reproducing strains as well as in the gcrmtrack of
bisexual species. Blending inheritance as originally conceived was
shown not to occur, and cytoplasmic inlieritance to play an
extremely subsidiary role.

The Mcndclians also found that mutations could be of any
extent, and accordingly that apparendy continuous as well as
obviously discontinuous variation had to be taken into account

26 EVOI.0TION: THE MODERN SYNTHESIS

in discussing heredity and evolution. The mathematicians found
•hat hiomcrric methods could be applied to ireo-mendehaii
postulates, and then become doubly fruitful. Cytology became
intimately linked with genetics. Experimental embryology and
the study of growth illuminated heredity, recapitulation, and
paleontology. Ecology and systematics provided new data and
new methods of approach to the evolutionary problem. Selec-
tion, both in nature and in the laboratory, was studied quanti-
tatively and experimentally. Mathematical analysis showed that
only particulate inheritance would permit evolutionary change :
blending inheritance, as postulated by Darwin, was shown by
R. A. Fisher (1930a) to demand mutation-rates enormously
higher than those actually found to occur. Thus, though it may
still be true in a formal sense that, as such an eminent geneticist
as Miss E. R. Saunders said at the British Association meeting
in 1920, “Mendelism is a theory of heredity: it is not a theory of
evolution”, yet the assertion is purely formal. Mendelism is now
seen as an essential part of the theory of evolution. Mendelian
analysis does not merely explain the distributive hereditary
mechanism: it also, together with selection, explains the pro-
gressive mechanism of evolution.

Biology in the last twenty years, after a period in which new
disciplines were taken up in turn and worked out in comparative
isolation, has become a more unified science. It has embarked
upon a period of synthesis, until to-day it no longer presents the
spectacle of a number of semi-independent and largely contra-
dictory sub-sciences, but is coming to rival the unity of older
sciences like physics, in which advance in any one branch leads
almost at once to advance in all other fields, and theory and
experiment march hand-in-hand. As one chief result, there has
been a rebirth of Darwinism. The historical facts concerning this
trend are summarized by ShuU ih a recent book (1936). It is
noteworthy that T. H. Morgan, after having been one of the
most extreme critics of selectionist doctrine, has recently, as a
result of modem work in genetics (to which he lias himself so
largely contributed), ^ain become an upholder of the Darwinian
point of view (T. H. Morgan, 1925, and later writings); while

THE THEOHY OF NATURAE SELECTION A?

his yoimgcr colleagues, notably Muller and Sturtevant, arc
strongly selectionist in their evolutionary views.

The Darwinism thus reborn is a modified Darwinism, since it
must operate with facts unknown to Darwin; but it is still
Darwinism in the sense that it aims at giving a naturahstic inter-
pretation of evolution, and that its upholders, while constantly
striving for more facts and more experimental res»’lts, do not,
like some cautious spirits, reject the method of deduction.

Hogben (1931, p. 145 seq.) disagrees with this conclusion. He
accepts the findings of neo-Mendelism and the mathematical
conclusions to be drawn from them; but, to use his own words,
“the essential difference between the theory of natural selection
expounded by such contemporary writers as J. B. S. Haldane,
Sewall Wright, and R. A. Fisher, as contrasted with that of
Darwin, resides in the fact that Darwin interpreted the process of
artificial selection in terms of a theory of ‘blending inheritance’
universally accepted by liis own generation, whereas the modern
view is based on the Theory of Particulate Inheritance. The
consequences of the two views are very different. According to
the Darwinian doctrine, evolution is an essentially continuous
process, and selection is essentially creative in the sense that no
change would occur if selection were removed. According to the
modem doctrine, evolution is discontinuous. The differentiation
of varieties or species may suffer periods of stagnation. Selection
is a destructive agency.”

Accordingly, Hogben would entirely repudiate the tide of
Darwinism for the modem outlook, and would prefer to see the
term Natural Selection replaced by another to mark the new
connotations it has acquired, although on this latter point he is
prepared to admit the convenience of retention.

These objections, coming from a biologist of Hogben’s calibre,
must carry weight. On the other hand we shall see reason in
later chapters for finding them ungrounded. In the first place,
evolution, as revealed in fossil trends, is “an essentially continuous
process”. The building-blocks of evolution, in the shape of
mutations, are, to be sure, discrete quanta of change. But firstly,
the majority of them {and the very great majority of tho.se which

28 evolution: the modern synthesis

survive to become incorporated in the genetic constitution of
living things) appear to be of small extent; secondly, the e&ct
of a given mutation will be difierent according to the combina-
tions of modifying genes present (pp. 68 seq.); and thirdly, its
eSect may be masked or modified by environmental modifi-
cation. The net result will be that, for all practical purposes, most
of the variability of a species at any given moment will be
continuous, however accurate are the measurements made; and
that most evolutionary change will be gradual, to be detected
by a progressive shifting of a mean value from generation to
generation.

In the second plare, the statement that selection is a destructive
agency is not true, if it is meant to imply that it is werely destruc-
tive. It is also directive, and because it is directive, it has a share
in evolutionary creation. Neither mutation nor selection alone
is creative of anything important in evolution; but the two in
conjunction arc creative (p. 475).

Hogben is perfectly right in stressing the fact of the important
differences in content and impheation between the Darwinism
of Darwin or Weismann and that of Fisher or Haldane. We may,
however, reflect that the term atom is still in current use and the
atomic Aeory not yet rejected by physicists, in spite of the
supposedly indivisible units having been divided. Tiiis is because
modem physicists still find that the particles called atoms by their
predecessors do play an important role, even if they are compound
and do occasionally lose or gain particles and even change their
nature. If this is so, biologists may with a good heart continue to
be Darwinians and to employ the term Natural Selection, even
if Darwin knew nothing of mendelizing mutations, and if
selection is by itself incapable of changing the constitution of a
species or a line.*

It is with this reborn Darwinism, this mutated phoenix risen
from the ashes of the pyre kindled by men so unlike as Bateson
and Bergson, that I propose to deal in succeeding chapters.

* It should be added that Hoghen was in 1931 concerned to stress mutation-
pre.ssure as an agency of change — than a new and nor generally accepted
conception, .Since then he has allowed much more weight to the joint role

selection and mutation in producing adaptive change (see Mogben 1940).

CHAPTER 2

THE MULTIFORMITY OF EVOLUTION

1. The heterogeneity of evolution 29

2. The paleontological data , . . . . p. 31

3. Evolution in rare and abundant species . , . . p. 32

4. Adaptations and their interpretation p. 34

5. Adaptation and selection P- 37

6. The three aspects of biological fact . . . . . P- 40

7. The main types of evolutionary process , . , , p. 42

I. THE HETEROGENEITY OF EVOLUTION

With the reorientation made possible by modern genetics, evo-
lution is seen to be a joint product of mutation, recombination,
and selection. Contrary to the views of the Weismann school,
selection alone has been shown to be incapable of extending the
upper limit of variation, and therefore incapable by itself of
causing evolutionary change. Contrary to the views of the more
extreme mutationists and the believers in orthogenesis, mutation
alone has been shown to be incapable of producing directional
change, or of overriding selective effects. The two processes arc
complementary. Their interplay is as indispensable to evolution
as is that of hydrogen and oxygen to water. And, as wc shall
see in detail later, the third process, of recombination, is almost
equally essential, not only for conferring plasticity on the species
and allowing for a sufficient speed of evolutionary clij^ngc, but
also for adjusting the eiffects of mutations to the needs of the
organism.

In this book I shall endeavour to analyse some of tlic main
types of evolutionary change in terms of tliis dual responsibility,
and then, to disentangle the various main roles (for they arc
numerous and diverse) of selection. Tliis analysis will lead finally
to a discussion of the problem of evolutionary progirss — whether
any such process exists, whether it is explicable on selectionist

evolution: the modern synthesis

terms, and whether there is any prospect of its future contmuance
In the first place, then, evolution is an alarmingly large md
varied subject. The students of a particular aspect of evolua^
are prone to think that their conclusions are gener^y apphcabK
Sas in most cases they are not. The paleontobgist.
long evolutionary senes and claim that evolution is . , ^
gradual and always along a straight course, wbch may be either
fdaptive or non-adaprive. However, as Haldane (i 932 «)
pobted out, their conclusions apply almost
and to animals wWch are mosdy of marme type and all belongmg
to abundant species. In some land plants, on the contrary, we now
have evidence of a wholly different method of evolution, namely,
die discontinuous and abrupt formation of new species. Md in
rare forms, as Sewall Wright (1932) and Haldane (1932a)
especially have stressed, the course of evolution, or at least of
si^cific and generic evolution, vnll not run in the same way as

in abundant and dominant types (see also p. 3 87)-

Meanwhile the comparative physiologist and a certam type of
naturalist will inevitably be struck by the adaptive char^ters
of animals and plants: organisms are seen by them as bundles of
adaptadons, the problem of evolution becomes synonymom with
the problem of the origin of adaptation, and natural selection
is erected into an aU-powerful and all-pervading^ agen^. THs
was the orthodox post-Darwinian view up to the end of the
nineteenth century, as represented by Darwin himself in such
boob as the Fertilization of Orchids, by Wallace in his Darwinism,
by Weismann in The Evolution Theofyy by Poukon in The

Colours of Animals. 1 • 11

The systematist, on the other hand, and often the ecologically

minded naturalist, struck by the apparent uselessness of the
characters on which they determine species and genera, are apt
to overlook other characters which are adaptive but happen to
be of no use in systematics, and to neglect the broad and obviously
adaptive characters seen in larger taxonomic groups and m
paleontological trends. The result, as recorded for instance in
Robson and Richards’ book. The Variation of Animals in Nature
(1936), and Robson’s work on The Species Problem (1928), is

XHB MUXTIFORMIXY OF ' EVOLUTION

an , undue, belittling of the role of selection in evolution, and an
ovei'-eiiiphasis ,011 the origin of species as the key problem ol"
evolutionary biology.

The paleontologist, confronted with liis continuous and: long-
range trends, is prone to misunderstand the iinpiicatioiis oi a
discontinuous theory of change such as mutation, and to invoke
orchogcncsis or lamarckism as explanatory agencies. Since there
arc more rare than abundant species, the biogeographer will have
to discount the fact that he is dealing mainly with processes irre-
levant to the major trends of evolution regarded as a long-range
process; while the ecologist and the pure physiologist, appalled
by the complexity of the phenomena which they study, are apt
to give up the quest for any evolutionary explanation at all

2. THE PALEONTOLOGICAL DATA

We may perhaps take up these points a little more in detail.
There is first the point of the unrepresentative nature of the
material upon which the paleontologist relies. The chief groups
which have yielded detailed results of past evolutionary change
by means of fossils are the molluscs, the echmoderms, the brachio-
pods, the graptolites, and the trilobites. Among the vertebrates
we have, of course, numerous important fossils which reveal the
past History of the phylum; but for the most part they serve
merely to show the general course of evolution and the broad
relationsliip of the various groups: this is so with the famous
ArchaeopteryXyWith the extinct orders of reptiles, with the rep-
tilian forms ancestral to mammals, with the ostracoderms, whose
primitive structure has been revealed by the work of Steiisio.
Only in the placental mammals, however, and notably in the
horses, the titanothercs, the elephants, and one or two other lines,
do we meet with an abundance of fossil forms sufficient to give
us what wc may call (remembering the words of the psalmist,
“a thousand years in thy sight arc but as yesterday’’) the day-to-
day progress of evolutionary change. And even here the abun-
dance and the consequent detailed accuracy of the record arc less
than in the other groups mentioned.

32 EVOiUTIONt THE MODERN SYNTHESIS

All these others are aquatic and almost exclusively marine. The
graptolites ;ind trilobites endured only for a short period of the
geological record. So, among the molluscs, did die ammonites
and (so far as abundance and fossil preservation arc concerned)
the iiautiloids. The lamelhbranchs, on the other hand, and the
sea-urchins and starfish among the ecliinoderms have remained
abundant up to the present. This, however, does not appear to
matter. In all cases where fossils are abundantly preserved over
a considerable period, we find the same phenomena, The change
of form is very graduaL It is often along similar lines in related
types. And in general it appears that different characters vary
independendy: at any one horizon, for instance, the fossil sea-
urchins of the genus Micraster include a few specimens showing
characters reminiscent of the average of the horizon before, a few
with the same characters anticipating the average of the horizon
next deposited; but in general the average development of the
various diagnostic characters will be nearly constant, though there
is no rigid correlation and many specimens will show some
characters in advance of and others behind the mean for the
particular time (Hawkins, 1936). A similar state of affairs has
been found in the history of the horses (Matthew, 1926).

As showing the restricted nature of the material on which the
paleontologbt relies, it may be mentioned that Professor Hawkins
(1936), in his presidential address to the Geology Section of the
British Association at Blackpool, drew very far-reaching con-
clusions as to the method and course of evolution on the basis
of echinoderms, molluscs, and brachiopods alone. No trends
in vertebrates and no trends in land animals were discussed
by him.

3. EVOLUTION IN RASE AND ABUNDANT SPECIES

It is furthermore obvious that only abundant and widespread
species will be of any service in tracing the detailed course of past
evolution. Now there are various peculiarities distinguisliing rare
from abundant species. In the first place, abundant species will
have a larger reservoir of inheritable variation, botli actual and

THE MUITIFOHMITY OF EVOLUTION

potential. This can be deduced on theoretical lines from what wc
know of mutation (Wright, 1932). In addition, it has been
demonstrated as a fact in several cases. Darwin, on the basis of
qualitative inspection, asserted that it was so. And R. A. Fisher,
using all the apparatus of biometric and probability technique,
has now demonstrated that it holds for such diverse cliaracters
as the' colour of moths’ wings and the dimensions of birds’ eggs
(Fisher and Ford, 1928; Fisher, 193 7'*)- This will obviously confer
upon abundant species a greater evolutionary plasticity, a higher
potency of adaptive change.

Rare species, on the other hand, •wiU not only possess l«s
evolutionary adaptability, but will, as Sewall Wright (1932) has
emphasized, be prone to have useless or even deleterious muta-
tions become accidentally fixed in their constitution. When
numbers are increasing after being abnormally low, a chance
mutation may spread through a considerable proportion of the
population (p. 61). Further, genes which are neutral or even
deleterious have a chance of becoming incorporated in a small
local population-unit. Such “accidental” divergence may con-
tinue to an indefinite extent. Furthermore, rare species wiU tend
to become subdivided into discontinuous groups, and these,
once isolated, wiU have a greater likelihood of differentiating into
separate species, partly by the accidental accumulation of muta-
tioB, as we have just seen, and partly because selection can work
g on tbm unhampered by immigration from otlier areas inhabited
^ by slightly different types. Haldane (l932(j) draws attention to the
f fact that the rare fern Nephrodium spimlosum has no fewer than
i four weU-marked local subspecies (or even species, according to

i some authorities) in isolated areas of Britain alone.

Many abundant species, on the other hand, will differentiate
into subspecies in different parts of a continuous range; these wUl
differ adaptively in accordance with the environment, but there
wUl not be complete isolation between them (except as the result
of climatic or geological change producing a barrier) and migra-
tion ■wUl keep distributing genes from one subspecies to its
neighbours (Chap. 5, § 3).

When this is so, SewaU Wright (1931) points out that the

evolution: the modern synthesis

variability of a species will be at a maximtim; for the agency of
selection will have added partial local differentiation to the
intrinsic variability of a large population, and migration will be
ensuring new recombinations of the genes determining sub-
specific characters.

In abundant plant species, the chief tendency appears to be to
differentiate into numerous ecotypes, many of which will co-exist
in the same geographical region (pp. 275 seq.). These too will be
able to exchange genes, and thus to promote variability.

Furthermore, as Haldane (i932<j) has stressed, competition and
therefore selection in rare species will be more l^tween the
species as a whole and its environment, or between it and other
related species, while in abundant species they will be more
between individual members of the same species. And this intra-
specific selection has various peculiar results in evolution, many of
them in the long run being harmful to the stock (p. 478).

We must also mention the interesting work of Willis, summar-
iaed in his book. Age and Area (1922). In the first place, he points
out that rare species are more numerous than abundant species.
For instance, out of 809 species of flowering plants in Ceylon,
65 per cent arc rare, including 37 per cent “extremely rare”
(sec p. 204). His figures apply chiefly to flowering plants, but even
a casual acquaintance with systematics makes it clear that a
situation of this type is general, and that thesystematist and biop^^
grapher arc dealing with many more scarce than abundant

known in several groups of flowering plants, and may w
prove to be considerably commoner than is now gencrah
supposed, save by a few cytologists and geneticists.

4. ADAPTATIONS AJMD THEIE INTERPSETATION

On the subject of adaptation, also, I shall have more to say in a
later chapter. But it is clear that, whatever value we allow to the
deductive method and its implications as regards adaptation, it

The discontinuous formation, per saltum^ of new specie^^^^
plants I shall treat of later in detail, in connection with the
problem in general. Suffice it to say here that the phenomeno?^

THE M0LTIFORMITY OF EYOtUTION

must not be alowed too free a rein. Specolation must be constantly
cbccked by obser¥ation and experiment.

A striking example of this comes from recent work on sexual
selection (see summaries in Huxley, 1938a, 1938!?, I938r). Under
the impetus of Darwin’s great work. The Descent of Man,, what
may be called the orthodox Darwinian view came to be generally
held, namely, that all bright colours of higher animals which are
restricted to the male sex, are, in the absence of deiSmte evidence
to the contrary, to be interpreted as owing their origin to sexual
selection; the same was assumed for the songs of birds. When
these bright colours were known to be conspicuously shown oifF
in some special display attitude, the conclusion was regarded as
incontrovertible; but even when this was not the case, as with
most of the bright colours of male ducks, the presumption was
regarded as sound. It was rather the opposite of the presumption
of British law that a prisoner is to be regarded as innocent until
deiBnite proof of guilt is adduced. In reaction against this attitude,
however, many biologists adopted an equally uncritical attitude
of scepticism, and many even proclaimed that sexual selection
had been '^disproved” and that no masculme colour or other
characters had any function in stimulating the female.

However, while this scepticism is wholly unjustified in fece
of the vast body of positive evidence, notably from field study,
recent work, both observational and experimental, largely on
birds but also on lizards (see Noble and Bradley, 1933), has
shown that the Darwinian presumption in its sweeping form was
erroneous. Only when the bright colour or other performance is
solely or mainly used in display before the female can it hold.
If so, however, the presumption is very strong that its origin is
to be sought in sexual selection in the modem sense, which Offers
considerably from that in which it was originally employed by
Darwin, and the burden of proof is on the other side. . :

Song, on the other hand, as a result of detailed observation, is
now regarded as having its prime fun.ction as a ‘‘distance threat”
to rival males and its secondary function as an advertisement, so
long as the singer is onmated, to unmated females. The same is
true of bright colours in the males of territorial species. Similarly

36 evolution: THE MODERN SYNTHESIS

bright colours have in many cases been proved to have the
function of simple threat and not that of display. The most
striking case is perhaps that of our common robin {Erithacus
mheculd). It was known that the bright red colour of the breast
is actually displayed very prominently in a special stiff erect pose
during the breeding season, and it had been generally assumed that
this was a display of the male towards the female. The observ-
ations of Burkitt (1924-5), however, and of Lack (i939)> and
experiments with stuffed birds have shown that this pose is one
of threat and is used by members of either sex, but exclusively
towards territorial rivals. It is noteworthy that in the robin,
both sexes hold territories in autumn and early winter, so
that the marked development of threat action and threat colour-
ation in die female as well as the male has an obvious adaptive
significance.

Observation again has shown that one and the same colour or
structure may be employed in different ways as a threat to rivals
or as a display to potential mates — ^this holds for blackcock and
ruffs, for instance ; while in other cases, as in the train of the
peacock, the display funaion appears to be the only one. Thus
deductive speculation, though legitimate in its place, must be
closely checked.

Precisely similar considerations apply to all other cases of
adaptation. For instance, elementary observation shows a corre-
lation between the prevailing colour and pattern of animals and
that of their environment. This provides a prirnafade case for the
relation being an adaptive one. But this deduction is a first
approximation only. The next step is to make detailed ecological
observations on particular cases; to see whether alternative
explanations may not be preferable (such as the view that there
is a direct effect of the colour of the environment on the organism,
or an indirect effect via the prevailing climate; see Dice a”nd
Blossom, 193 7 f for a' case- where the climatic interpretation can
be rejected); and, where possible, to check the adaptive value by
experiment. We sliall then be able to reject a certain number of
suggestions (such as that of Thayer (1909) that the pink colour
of flamingos enables them to escape detection against the sunset

THE MULTIFOSMITY OF EVOLUTION 37

sky), and to retain a certain body of firmly establish^ fact and a
considerable residuum of high probability.

We need not be deterred by arguments of a negative nature,
such as that which maintains that a particular arrangement cannot
be adaptive, because related species do not show it; for these can
be shown to rest on a lack of biological logic {p. 466).

On the basis of such a step-by-step analysis, we shall obtain
strong support for the view that adaptation is all-pervading and
of major importance, even if it does not apply to numerous
details of the structure and function of organisms. And this wili
enable us to discount non-adaptive theories of evolution, such" as
ortho^nesis, as being based either on incomplete data or on
deliberate rejection of the adaptational point of view.

5. ADAPTATION AND SELECnON

Finally, another and even more important point of method must
be mentioned. It concerns the types of conclusions which can be
drawn from diSerent types of data. I will begin with an example.

Various writers, naturally comprising a number of pale-
ontologists, have advanced views on genetics and selection, which
are based upon the data of paleontology. For instance, some have
claimed that Lamarckian theories of inheritance and evolutionary
change must be true, since paleontological change is in the
majority of cases of a functional nature, suiting the stock pre-
gressively to a particular mode of life. For instance, MacBride
(1936), after reviewing certain evidence for the inheritance of
habit, continues : “When fully documented evidence for evolution
as displayed by a minute study of species and races of living
forms or by the study of lineage series in fossils is carefully
studied, this dependence of evolutionary change on change in
habit and function becomes apparent.” And .he draws the further
conclusion that the changed habit or function is the direct cause
of the evolutionary change.

Others, while not going so far in a positive direction, insist
that any selective theory based upon inheritable variation occur-
ring at random, or at least in many directions, cannot be true,

38 evolution; THE MODEBN SYNTHESIS^^^^ ^

The reason alleged is that the fossil record shows nothing of this
randomness, but always advances along definite directions. Still
others, more impressed by this fact of direction, and by the
further fact that the directional change does not always seem to
be functional, but may be of an apparently useless or even
deleterious nature, assert not merely that selection cannot be
responsible, but that the prime cause of evolution must be the
inner momentum which in technical parlance is called orthogenesis
(see c.g. Hawkins, 1936, and Chap. 9 of this book).

Quite bluntly and simply, all such assertions are unjustified.
They are unjustified on the score of simple logic and scientific
method. Paleontology is of such a nature that its data by them-
selves carmot throw any important light on genetics or selection.
As admitted by various paleontologists (e.g. Swinnerton, 1940),
a study of the course of evolution cannot be decisive in regard
•TO the method of evolution. All that paleontology can do in this
latter field is to assert that, as regards the type of organisms which
it studies, the evolutionary methods suggested by the geneticists
and evolutionists shall not contradict its data. For instance, in
face of the gradualness of transformation revealed by paleontology
in sea-urchins or horses it is no good suggesting that large
mutations of the sort envisaged by de Vries shall have played a
major part in providii^ the material for evolutionary change.
Even here, however, let us be careful to note the restriction
imposed by the phrase “as regards the type of organisms which
it studies”. The main lines of evolution in the more abundant
forms of sea-urchins, horses, and the like may depend upon
gradual change: but this is no reason for assuming that this holds
for all organisms. And as a matter of fact, as we shall set forth
more in detail later, abrupt changes of large extent do play a part
in certain kinds of evolution in certain kinds of plants.

It may be worth while to see why and how the assertions,
positive and negative, that we have just been commenting on,
are methodologically unjustified. In the first place to state that
the functional nature of evolutionary change presupposes a genetic
mechanism like that postulated by Lamarckism, which involves
the inheritance of modifications in the individual brought about

THE MULTIFORMITY OF EVOLUTION

by use and function, is a non sequitur. A functionaliy-guided course
(if evolution is conso;iant with a lamarckian method for evo-
lution but it is also consonant with an anti-lamarckian Darwin-
ism. For the natural selection of “spontaneous” variations which
in their origin have nothing to do with the eSbet of use or disuse,
provides a perfectly adequate formal explanation of the genesis
of organisms adapted to their mode of life, and therefore of a
functionally-guided course of evolution. The difference lies in
the intermediary steps: in the one case the effect of use or func-
tion is supposed to be direct, in the other indirect, via the sifting
mechanism of selection.

There is thus a non sequitur in the fundamental postulate of
functionally-dircctcd fossil lineages presupposing lamarckian
evolution. MacBride {e.g. 1936), however, goes even further.
He imphes that all evolutionary change is functionally deter-
mined. But in the first place we shall later describe certain trends
revealed by paleontology, notably in ammonites and lamelli-
branchs, for wliich no functional explanation has so tar been
suggested (p. 506). And, in the second place, the evidence on the
differences between allied species, as collected by such authors as
Robson and Richards (1936), indicates that many specific

characters are non-adaptive. Even if we discount many of these
as being in all probability useless consequences or correlates of
useful charaaers, a residuum remains. Thus here again we find
are brought out the multiformity of evolution and the impossi-
bility of ascribing all kinds of evolutionary change to a single
mechanism.

Similarly the argument that straight-line or directional evolu-
tion as revealed by paleontology rules out the natural selection
of random variations is simply not true. On the postulate of
natural selection, the overwhelming majority of the individuals
which survive will clearly be of the adapted type. The likelihood
of any obviously maladjusted types surviving to become fossilized
is negl^ble. Further, at any one moment, if there is a constant
pressure of selection, and if the raw material on which it
acts is constituted by small mutations, as appears to be the case
(PP- 5i» 58 n.), the main alteration of the stock will be brought

40 evolution: the modern synthesis

about by the slightly lower survival- and reproduction-rate of
the types which, though already broadly adapted, are not so
highly adapted as others. Thus for the most part the constitution
of the stock, as revealed in the bulk of those individuals which
reach maturity, will change by a gradual increase of more highly
as against less highly adapted types, not by the selection of “the
adapted” as against “the non-adapted”.

Again, directional evolution does not necessitate orthogenesis,
since, so long as it is functional and adaptive, natural selection
will also provide a formal explanation of it. An orthogenetic
theory will be necessary if studies on m utation show that mutation
(a) is so frequent that it can override selective influences and
(h) if it also tends to occur repeatedly in the same direction. It
will also be necessary to account for directional evolution which
is useless or deleterious, or is not correlated with adaptive func-
tion. These points we shall discuss later (pp. 504 seq.).

6. THE THREE ASPECTS OF BIOLOGICAL FACT

If we look at the matter in the most general light, we shall sec
that every biological fact can be considered under tltree rather
distinct aspects. First, there is the mechanistic-physiological aspect:
how is the organ constructed, how docs the process take place ?
Secondly, there is the adaptive-functional aspect: what is tlie
functional use of the organ or process, what is its biological
meaning or value to the organism or the species t And in the
third place, there is the historical aspect: wliat is die temporal
history of the organ or process, what has been its evolutionary
course? A couple of examples will illuminate the point. The
auditory ossicles or small bones of our middle ear operate so m
to transmit vibrations of the ear-drum' to the fluid on the inner
ear. Their functional significance is to enable us to hear. And
historically they are derived from the inner portions of the upper
jaw, the lower jaw, and the hyoid, wliich have changed their
function in the course of evolution. Or again, the notochord,
which appears traiLsitionally in the development of all higher
vertebrates, is historically a recapitulation of the stage when all

THE MULTIFORMITY OF EVOLUTION 41

ancestral vertebrates possessed no backbone but a notochord
persistent through life. Mechanistically, it is developed by a
process of self-determination from the central portion of tlie
invaginated dorsal Hp of the blaostsporc. Functionally, it appears
to serve as a temporary scaffolding around winch the true back-
bone may afterwards be most conveniently laid down.

Sometimes a character may possess no present functional value,
and can only be understood in the light of its evolutionary past.
This appears to be true for the hind-Hmb bones of whales or the
vestigial hairs on our own bodies. But in all cases the three aspects
are distinct; each must be investigated separately by appropriate
methods, which may have no relevance to the other aspects;
and discoveries concerning any one aspect can only be of limiting
nature, and not decisive or essential, with regard to the other two
aspects. They represent three separate fields of discourse, which
may overlap, but are of fundamentally different natures.

These considerations apply to evolution as to all other biological
phenomena. Paleontology deals with the historical course of
evolution. The machinery -for the transmission of hereditary
factors, together with any differential survival or reproduction
of individuals of different types, constitutes the mechanism of the
process. The adaptations of species or evolutionary lines, and the
reasons for their spread or their extinction, constitute the func-
tional aspect.

We have seen the illegitimacy of using data on the course of
evolution to make assertions as to its mechanism; but the con-
verse is just as indefensible. For instance, as we have already said,
the assumption of the de Vriesian mutationists that discontinuous
variations of large extent are the main source of evolutionary
change is not consonant with the facts revealed by paleontology.
Again, the demonstration that small mutations occur and can
serve as the raw material on which natural selection may act to
effect gradual evolutionary change, docs not mean that this is
necessarily the only type of evolutionary change possible. We
have already mentioned that species may be formed abruptly, and
other large variations arc known to occur and to serve, in some
cases, as building-blocks of evolutionary change (Chap. 6, §§ 8 , 9 ).

42 evolution: the MOBERN

The consideration of evolution thus demands data from the
following branches of biology. As regards its historical course,
direcdy from paleontology and indirectly from systematics and
biogeography. As regards mechanism, from genetics and cytol-
ogy, and, since the expression of a gene is important, froin
studies of development and growth; in addition, systematics
may throw light on the types of variation to be found in nature.
And as regards biological meaning, from physiology and ecology
for the study of adaptation; from mathematics, selection experi-
ments, and, indirectly, from paleontology, for the study of
survival and extinction. All these are necessary, but none of them
alone is sufficient.

7 . THE MAIN TYPES OP EVOIUTIONAEY PROCESS

If v e may anticipate some of the results of later chapters, we may
summarize our conclusions briefly as follows. Evolution in
biology is a loose and comprehensive term applied to cover any
and every change occurring in the constitution of systematic
units of animals and plants, from the formation of a new sub-
species or variety to the trends, continued through hundreds of
millions of years, to be observed in large groups.

The main processes covered by the term are as follows,
(i) Long-continued trends, as revealed by indirect evidence and
in some cases by the immediate data of fossils. These are for the
most part towards spedalization (p. 486), a number of them
towards that peculiar form of specialization called degeneration
(p- 558), and a few towards that all-round biological improve-
ment which may be styled evolutionary progress (p. 5 59 )- AH
these are essenti^y adaptive, or, if you prefer it, functionally
guided. In addition, certain trends occur which cannot be inter-
preted adaptively, at least in the Mght of present knowledge, such
as that of various hnes of ammonites to greater complexity
followed by progressive unrolling of the spiral and by other
simplifications (p. 506).

Indirect evidence for similar trends, at least for those of
adaptive type, is provided by comparative anatomy and embry-

THE MULTIFORMITY OF EVOLUTION 43

ology^ When a group is considered as a whole, it will be found
in the early stages of its history to be -.adiating into a number of
trends; in other words, its evolution is essentially divergent.

An important comphcation is provided by the fact that selection
may have quite different effects according to the group of indi-
viduals on which it acts. Thus selection in social insects like
bees and ants in which most individuals are neuters and repro-
duction is concentrated in a small special caste, can produce
characters in the species of quite a different type from those
possible to animals of the usual type, in which all individuals are
capable of reproduction {p. 482). Again, in higher mammals, the
fact that the mother nourishes a litter within her body will lead
to a special type of selection acting upon the unborn young, and
this will have repercussions on die evolution of the species
(p. 525). Similarly, the competition between pollen-grains in
Bgher plants leads to a type of selection which is absent in higher
anitnaU (p. 481), while the necessity for intemal fertilization in
bi glifr animals has led to the type of selection, with characteristic
effects, kno-wn as sexual or inter-male selection (pp. 425 seq.).

Again, the existence of growmg-points and other regions of
permanendy embryonic tissue in higher plants gives them oppor-
tunities for asexual reproduction and for taking advantage of
mutations that are denied to higher animals.

(2) Minor systematic changes, as revealed by detailed tax-
onomy, ecology, cytology, and genetics. When we come to
minor systematic change, we find some very different processes
at work. Some processes of spccies-diflKrentiation will, of course,
form part of a major trend, whether by the direct evolution of a
species into an altered form or by its divergence into two lines
of incipient specudization. But many processes involving the
formation of species and subspecies will be of a different character
Plant species may be produced discontinuously so that no
selection is involved in their formation, but only in their subse-
quent fate (p. 340), A large species may become broken up into
^ghtly difierentiated subspecies, each somewhat adapted to local
conditions, but interbreeding to a certain degree with neighbour-
ing species. If really small local groups arc wholly isolated firom

44 evolution: the modeen synthesis

interbreeding with the rest of the species, their total variabihty
will be insufficient to respond readdy to selection, useless or even
deleterious characters due to chance (pp. 199 seq.) may crop up
in them, and they will be more prone than larger groups to
become extinguished when conditions alter. The same is true of
once numerous species which dwindle until they become small.

In certain conditions, as on the Galapagos archipelago, the
few immigrants which have succeeded in reaching the place
have blossomed out into an extraordinary array of species;
it seems diat local isolation, coupled with absence of biological
competition, is involved (see Swarth, 1931, 1934, and Chap. 6,
§7 of this book).

It may be presumed, on somewhat indirect evidence, that
“useless” non-adaptive differences due to isolation of small
groups may be enlarged by the addition of further differences of
die same sort to give generic distinction, though it seems prob-
able that differences of family or higher rank are always or almost
always essentially adaptive in nature.

As we shall discuss more fully later (p. 478), both competition
and therefore selection in abundant species are mainly intra-
specific, between individuals of the same spedes; while with rare
spedes they are mainly interspecific, between the species as a
whole and its rivals, or as a struggle of the spedes as a whole to
survive in its changing local environment.

A spedes, besides becoming differentiated into local subspedes,
may show polymorphism. In some cases, as with various
mimetic butterflies, the difierent forms are highly adaptive and
differ in many details, while in other cases, as with the existence
of two or more colour-phases {e.g. in black and red squirrels,
or pale and normal clouded yellow butterflies; pp. 96 seq.), the
forms differ in some simple mcndclian character and their adaptive-
significance is not at all obvious. In some such cases certain by-
products of the mcndclian mechanism of heredity and mutation
seem to be responsible for permitting this sharp polymorphism to
occur, diough in others there is a selective balance, weighted
difierently in ecologically different parts of the organism’s range
(pp. 103 seq.).

THE MULTIFORMITY OF EVOLUTION

Again, in higher animals, mutual recognition may be at a
premium. The recognition may be of one member by all others,
as with the recognition marks of gregarious birds and mammals;
or it may be between members of opposite sex, as in colours,
sounds, or scents promoting the approximation or stimulating
the coition of the sexes; or between members of the same sex,
as with threat-characters. In most of these cases it is biologically
desirable to prevent confusion with similar characters of related
species occupying the same area (pp. 288 seq.). Recognition-
characten accordingly are in most cases not only striking but
strikingly different from species to species. Selection in these
conditions operates to produce distinctiveness — difference for the
sake of difference (see Lorenz, 1935, Huxley, 1938c).

In still other cases, frequently in plants and rarely in animals,
an interbreeding group (species or subspecies) has been produced
by crossing between two or more incipient or fuUy differentiated
species. The results differ according to the precise genetic and
cytological mechanisms operative (Chap. 6, §§ 8, 9); in some
cases, however, an abnormal degree of variability is generated.

All these different processes — the adaptive and the non-
adaptive trends, and all the various types of specific, subspecific,
and polymorphic divergences — are equally part of evolution. If
the long-range trends are in the long view the more important,
the minor changes probably concern a far larger number of
species. It would seem clear that we cannot expect to find a single
cause of evolution: rather we must look for several agencies
which alone or in combination will account for the very various
processes lumped together under that comprehensive term.

Looking at the matter from another angle, we are beginning to
realize that different groups may be expected to show different
kinds of evolution.

Only forms which are able to dispense entirely widi bisexual
reproduction will be able to establi^ new species by autopoly-
ploidy; the establishment of new species by hybridization and
allopolyploidy wiU in the mam be confined to forms with un-
limited growth of the type found in higher plants; purely apo-
mictic forms will show a host of slighdy dififerent pure lines;

46 EVOLUTION :■ THE ' MOBEEN ■ : SYNTHESIS''

aiunial groups with wide powers of dispersal like birds will tend
to “develop characters for sex-recognition and discrimination to
prevent intercrossing with other species; the type and amomt of
variation and diSerentiation will be different in cross-fertilizing
as against self-fertilizing or non-sexual forms, in fertile polyploids
as against diploids, in sedentary as against mobile forms (Chap. 4),
Just as there is no one method of the origin of species, so there
is no one type of variation* Different evolutionary agencies <hffer
in intensity and sometimes in kind in different sorts of organisms,
partly owing to differences in die environment, partly to (hffer-
ences in way of life, partly to differences in genetic machinery.
No single formula can be universally applicable; but the different
aspects of evolution must be studied afresh in every group of
animals and plants. We are approaching the time when evolution
must be studied not only broadly and deductively, not only
intensively and analytically, but as a comparative subject.

CHAPTER 3

MENDELISM AND EVOLUTION

r. Mutation and selection , p. 4J

2. Genes and characters . . , p. 62

3. The alteration of genic expression F-

4. The evolution of dominance F- 75

5. Types of mutation . . * p. 87

6. Special cases: melanism; polymorphism;

fluctuating populations ........ p. 93

7. Mutation and evolution * . . . . . . . p, its

I. MUTATION AND SELECTION

The essence of Mendelian heredity is that it is particulate. The
genetic constitution is composed of discrete units. Each kind of
unit can exist in a number of discrete forms. The hereditary trans-
mission of any one kind of unit is more or less independent of that
of other units, the restriction of independence being a partial one,
concerned with the phenomenon of linkage. The units are the
Mendelian factors or genes, while their different forms are called
allelomorphs or alleles.

The particulate nature of inheritance enables calculations to be
made as to the proportions of offspring of different types in
different generations after a cross. Like die atomic theory in
chemistry, it is the basis of quantitative treatment.

The hereditary particles or genes are located within the visible
chromosomes, whose manoeuvres distribute their contained
genes equally to all cells of the body, and determine the quanti-
tative details of Mendel’s laws. Within a particular chromosome,
each gene has its appointed place, which it keeps permanently
(apart from rare rearrangements; pp. 89 seq.). Of recent years, the
study of the giant chromosomes in the salivary glands has con-
verted Drosophila from an . unfavourable into an exceedingly
favourable cytological object. It is now possible in tliis genus to

48 EVOLUTXON: THE MODERN SYNTHESIS

check genetic prophecies cytologically and cytological prophecies
genetically, in a remarkably complete and detailed way. (For the
cytological basis of heredity see Darlington, 1937; briefer treat-
ment in M. J. D. White, 1937.)

It used to be imagined that the precise arrangement of the
genes within the dbxomosomes was biologically irrelevant.
To-day, however, we know that this is not so. Genes (all or many
of them) have somewhat diferent actions according to what
neighbours they possess. This is the so-called position effect
(p. 85), which, only recently discovered, will probably turn out
to be widespread, although in some organisms (such as maize)
it does not seem to occur. Where it occurs, it is likely to be of
fundamental importance as well, since the rearrangement of
blocks of genes (sections of chromosomes) within the chromo-
some outfit (p. 89), though considerably rarer than gene-mutation,
is not infrequent in the long persjx'ctive of evolution; and this,
through the position effects which it frequently causes, provides
a large and previously unsuspected source of variability and
potential evolutionary change. Its contribution, however, must
be much less varied and much less abundant than that of gene-
mutation (Muller, 1940).*

This does not affect the basic conception of the gene as particu-
late. Genes are in many ways as unitary as atoms, although we
cannot isolate single genes. They do not grade into each otlier:
but they vary in their action in accordance with their mutual
relations. In this they are again like atonts: the chemical behaviour
of a compound will be altered when we transfer an atom from
one position to another in the molecule, even though the sub-
stantive constitution of the molecule remains unchanged. Thus the
whole is not merely the sum ofits parts: it is also their relation.

The discreteness of the genes may prove to be notliing more

* A special case of position eflfect is the modification of variegation (mosaicism)
of various sorts in Drosaphlla^ excited on various genes if translocated into prox-
imity with heterochromatin (**incrt** regions of the chromosomes). Schuit2’s
summary (1941) makes it clear that this is caused by a change in the nudeic acid
metabolism of the translocated regions, and that this exerts its effects in early
stages of development, by causing a process akin to inactivation of the genes
involved. The degree of this inactivatk^n decreases with the distance from tlic
point of breakage and jc-union with the inert region.

^ ^ ^ ^ AND EVOIUTION 49

til an the presence of predetermined zones of breakage at small
and more or less regular distances along the chromosomes. For
the independent hereditary behaviour of genes, from which their
discreteness is deduced, is due to two facts. When the genes to
be tested are in different kinds of chromosomes, their independ-
ence is due merely to the independent behaviour of the two
chromosomes. But when they are both in the same kind of
chromosome, their independence depends on what is known as
crossing-over. Prior to the formation of reproductive cells, die
two homologous chromosomes of each kind pair together.
Where they touch, they may break and exchange segments; in
the daughter-chromosomes the kinds of genes and their order
remain as before, but one or more blocks of genes from one
chromosome will have exchanged places with precisely corre-
sponding blocks of genes from the other. The breaks do not
always occur in the same place. If there is more than one break
in a chromosome-pair, the second break is at a considerable
distance from the first; thus breaks can normally not occur on
both sides of a single gene. What happens is thus that genes are
separated from their erstwhile neighbours in a chromosome by
these breaks; and it is the fact that breaks may occur at different
places in the chromosome which makes it possible for any gene
in a chromosome to be separated from its neighbours and thus
to be inherited independendy. Knowing this, we may put the
matter the other way round, and say that the process of exchange
of sections after breaking, and the fact that breakage only occurs
at certain spots, determines what we call the gene (see discussion
in Griineberg, 1937) ; a gene-unit is thus a section of the chromo-
some between two adjacent sites of potential breakage at crossing-
over.*

The chromosomes may thus be looked on as “super-molecules”,
built up out of a series of regions, each region marked off by
zones of potential breakage. The portions of tihese regions which
wc can recognize by their efiects in inheritance are what we

* It is possible, though not fully established, that the breaks underlying sectional
rearrangements may not invariably coincide with the sites of potential breakage
underlying crossing-over. If so, sectional chromosome-mutations may actually
genes in two (Muller, 1940; Ralfel and Muller, i 94 o)*

so BVOiUTlON : THB MODBIH SlfNXHBSIS

Gal Rearrangement of the regions, as wel as change within

a single region, or the loss or duplication of a region, or set
regions, can and normaly wil came alterations in the action o
the chromosome and its parts on the developing organism.
Similarly the doubling of the whole chromosome-outfit, by pro-
ducing a different relation between gene-outfit and cytoplasm,
wil also alter the characters of the organism.

The number of genes is much larger than was originally
imagined. Drosophila is the only organism where adequate
quantitative knowledge is available. Several recent estimates,
based on different mediods of approach (sec summary in Gulick,
1938), vary from a niiiiimum of over 2,000 to a maximum of wel
under 13,000, vnth a probable number of about 5 >000, for this
minute insect. The size of a Drosophila gene must be between
io~® and 10*"^ ft® and probably between 10 and 10 ^
equivalent to some 10 medium-sized protein, molecules (see also
Lea, 1940). In some other organisms {Lilium) the genes may be
larger, and in others more numerous (e.g. in man, perhaps 4 to
6 times more so than in Drosophila). It will be seen what astro-
nomical possibilities of recombination and mutual interaction
are afforded by an assemblage of this magnitude.

A gene-mutation wll then be any intrinsic change in substance
or structure, affecting the mode of action of one of these unit-
regions.

One of the notable biological discoveries of the last few years
was that of MuUer, on the effect of X-rays in producing genc-
mutadons. The ordinary rate of mutation can by this means be
multiplied a hundred-and-fifty fold, and a certain number of
wholy new mutations, in addition to many already known, are
produced. It is, however, interesting, in view of our discussion,
to note that X-rays also induce the rearrangement of chromo-
some-sections by translocation, inversion, etc. In view of the
assertions of certain biologists that mutation is of its essence
pathological, it should be mentioned that X-ray treatment can
produce reverse mutation — ^i.e. cause a previously mutated gene
to revert to normal (Patterson and Muller, 1930; Timofeeff-
Ressovsky, I934«i, 1937), Comment is needless.

MBNDBLISM AKD EVOLUTION 51

Timakov {1941) in wild Drosophila has detected a gene increas-
ing mutation-rate at least 40 times, and possibly to a level higher
than that induced by X-rays.

The fact that the genes and their arrangement normally remain
constant, until altered by some kind of mutation (after which
they again remain constant in their new form until a further
mutation supervenes), accounts for the resemblance between
parents and offspring, The fact of Mendehan recombination, by
wliich new combinations of old genes are produced according to
Mendel’s second law (and to the rules of crossing-over), accounts
for the great majority of the differences between parents and
ofispring, and between members of a family or population. But
gene-mutation, though a rare event, appears to account for most
that is truly new in evolution. Under the head of gene-mutation,
position-effect due to very small sectional rearrangements can
legitimately be included, since it involves a structural cliange
and a novel effect; further, it caimot for practical reasons be
excluded, since there is at present in many cases no possibility of
distinguishing between it and true gene-mutation. Recombination
also may in certain cases produce evolutionary novelty, for
instance after a cross between two previously isolated types.
Finally, as we shall see later, hybridization, widi no subsequent
recombination, may sometimes be responsible for evolutionary
change (Chap. 6, § 9).

However, gene-mutations (including position-effects) appear
to be the most important source of novelty in evolution, and we
must now say a little more about them.

In the first place, no trace has been found in Drosophila, where
analysis has been pushed to an extraordinary pitch of refinement,
of any characters not dependent on chromosomal (and therefore
mendelizing) differences (Muller, 1940). Secondly, although
complete proof cannot be offered, the presumption, in the
absence of evidence to the contrary, is that all mendelizing gene-
differences owe their origin to mutation: up to the present we
know of no other way by which they could have come into
existence.

Finally, a mmibcr of mutations .are known which are roughly

52 evolution: THE MODERN SYNTHESIS

jicutral, or actually or potentially uschil (pp. i tH, 449). In barley,
Gustafsson (1941) induced by X-rays three mutations which
increased yield, one of tliem markedly. Among those which are
potentially useful a few cases may be mentioned here. Banta and
Wood {1928) discovered a “diermal race” of the crmtacean
Daphnia hngispina, which had its optimum 6°-8° C. higher and
its thermal death-point 5° C. higher than the long-established
parthenogenetic strain from which it originated. It was also
immobilked more quickly by low temperatures. Thus it was
potentially adapted to a warmer environment than its parent
strain. Its origin from sexually inbred individuals showed that it
depended upon a recessive mutation arising during the long
preceding period of parthenogenetic reproduction. Other more
immediately useful mutations arose in Ae same way, e.g. some
causing greater fertility and others greater longevity (Davenport,
1928).

Very important results were obtained by Johannsen (1913) in
following up his classical researches on pure lines for seed-
weight in beans. As is well known (see Johannsen, 1926), he
showed that the prime effect of selection in a mixed population
was to isolate pure lines, and tliat further selection then had no
further effect, in the absence of mutation. But he also showed that
mutations might occur in pure lines, and might then be selected.
During his experiments, two mutations, one for liigher and one
for lower seed-weight, were detected and “captured” by his
selection for high and low seed-weight respectively.

Anodier interesting case is that of the variety of tobacco
originated apparently by mutation, described by Gamer and
Allard (1920). This did not flower at all in its place of origin
(Washington); but when the daily period of light was reduced
to twelve hours, it flowered and set seed better than the stock
from which it had arisen. In other words, it was potentially better
adapted than the parent stock to more tropical latitudes.

SriU another case is the mutation described by McEwen (1937)
in Drosophila, which abolishes the fly’s phototropism. This
would be adaptive in dark surroundings. It is noteworthy that
this last effect is produced by the same recessive mutation that

MENDELISM AND EVOLUTION 53

changes the body-colour to tan: thus here a useless visible effect
is correlated with a potentially useful physiological effect (a
'^eorrelatcd character” in Darwin’s usage).

Ofmendclizmg differences, alike in domestic and wild species,
which are actually or potentially favourable, there is an abund-
ance. We need only think of the genes producing small and
large size in poultry (Punnett and Bailey, 1914) ; those producing
the specific differences between the two species of snapdragon
crossed by Baur (1923); the mimetic polymorphic forms of
various butterflies (p. loi); the diSerent forms of hetcrostyled
flowers such as primrose {Primula) and loosestrife (Lythrum ) ; the
single-brooded and double-brooded condition in silkworms;
and so on. An interesting case of a Drosophila mutant establishing
itself in considerable numbers in the wild is the vermiiion-cyed
type of D. hydei (Spencer, 1932). This mutant must be very
delicately balanced in its selective relations. The recent establish-
ment of other marked mutant types, like the black hamster, the
black Tasmanian opossum, the simplex-toolkcd field-vole, etc., are
discussed later (pp. 103-6, 203). In our own species, the work of
Blakesice and liis collaborators (see Blakeslee and Fox, 1932) has
established the existence of remarkable difierences, apparently
mendehan, in sensitivity of taste and smell in regard to various
chemical compounds and natural odours. These seem under
present conditions to be^ in themselves, somewhat selectively
neutral. Later work (Fisher, Ford and Huxley, 1939) has shown
that in chimpanzees not only are the same diflcrcnccs found, but
tasters and non-tasters occur in about the same proportions as
in man — close to 3 : i. This appears to indicate a stable balance
between the two conditions, and one depending upon some
advantage, of unknown nature, enjoyed by the heterozygotes.
The different blood-group genes would seem to fall into a some-
what similar category, though here the proportions vary markedly
in different populations: some of these genes occur also in various
lower mammals.

Many differences between “good” species have also been shown
to depend on mendclizuig gene-differences (see Goldschmid ,
1928, Chap. 15; Haldane, 1932^, Chap, 3; Laniprocht, 1941).

54 EVOtUTION: THE MODERN SYNTHESIS

Furdier, an increasing number of cbaracters once held to be
non-mendclian arc being shown to depend on mendelian genes
(e.g. the multiple factors influencing the hooded pattern in rats,
p. 65 ; the distinctive characters of wild subspecies of the deer-
mousc Peromyscus); and indeed wherever ¥2 shows greater
variability tha n Fi, inheritance must be particulate. Thus it may
be legitimately argued that the majority of all inherited characters
must rest on a mendelian basis. Even in the present incomplete
state of our knowledge, there are strong presumptive grounds for
this assertion, so that the onus of proof now lies on those who
would maintain the contrary in any particular case.

In addition, initially deleterious factors can be rendered use&l
by genetic-evolutionary methods which we shall discuss in
subsequent sections of this chapter.

Finally, mutations, while they seem to occur more readily in
certain directions than in others (Chapter 9), can be legitimately
said to be random with regard to evolution. That is to say, the
directions of the changes produced by them appear to be unre-
lated either to the direction of the evolutionary change to be
observed in the type, or to the adaptive or functional needs of
the organism. Evolutionary direction has to be imposed on
random mutation through the sifting and therefore guiding
action of selection. It is, of course, possible that as the laborious
technique of testing for mutation-rate is extended to more
species, certain mutations may be discovered which show very
much higher rates than others. However, the general agreement
already found between organisms so different as a monocotyle-
donous angiosperm, an imect, and a mammal would indicate
that in most species we may expect to find some mutations
occurring at a rate of i in jo® individuals or even higher, and
many genes with a mutation-frequency of about i in 10®. Occa-
sional genes with much higher mutation-rates occur (see summary
in Dobzhansky, 1937, Chapter 2), and some genes promote
increased mutation-frequency in other genes. In cotton, hybrid-
ization may increase the mutation-frequency of certain genes
(Harland, 1936). Mutation-frequency must in some way be
balanced against length of life; otherwise the chromosomes of

MENDELISM AN0 EVOLUTION 55

long-lived species would become crowded with. Icthals before
reproduction (Dobzhansky, 1937, p. 33). Again, the mutation-
rate for haemophilia in man is on the same scale as that for most
Drosophila genes if computed per life-cycle, but much lower
on the basis of time (Haldane, I935l>). Certainly species vary in
mutation-rate: thus the fern Nephrokpis exaltata has produced
many more mutants than any other species of the genus (Bene-
dict, 1931)- Zuitin (1941) in Drosophila finds that mutation-rate
is increased by sudden environmental changes (sec p. 1 37).

With mutation-rates of this order of magnitude, evolution
must always be a somewhat slow process, judged in terms of
years, but its speed in relation to geological time will be quite
adequate. R. A. Fisher (193 oa, 1932) has discussed the matter in
a general way. He clears the ground by pointing out that “blend-
ing inheritance,” which was currendy postulated in Darwin’s
day, would be constandy annulling variability: to be accurate,
the variance (in the absence of selective mating) would be halved
in each generation. As a result, new genetic variations (i.e. what
we should to-day call mutations) would have to be exceedingly
abundant — far in excess of anything observed in actual fact—
to produce die variance actually observed; and any variability
available for selection to act upon would have to be of very
recent origin. It was largely for these reasons that Darwin
ascribed so high an influence to “the direct effects of environ-
ment”.

The discovery that inheritance is almost entirely particulate
and non-blending removes these difficulties, so that in point of
fact the rise of Mendelism, far from being antagonistic to Dar-
winian views (as was claimed, notably by the early Mendelians
themselves, in the years immediately following its rediscovery),
makes a selectionist interpretation of evolution far simpler. In
mathematical language, it indefinitely conserves much genetic
variance instead of rapidly dissipating it, and thus amasses material
on which selection can work,* Further, if particulate inheritance

* A certain number of rare mutant genes will be lost to the species by
accidental elimination. In addition, genetic variance will be reduced by the
HJlccdvc dirnimtion of deleterious mutant genes.

56 evolution : THE MODEEN SYNTHESIS

and discontinuous mutation “as they are known at tiie present
time are the general basis of genetics and variation, selectionist
views also gain support over those strictly to be called ortho-
genetic (Chapter 9), in which the direction of mutation itself
is supposed to determine the course of evolution, and over those
to be called Lamarckian, in which the effects of use and function
arc supposed to be inherited. For no rate of hereditary change
hitherto observed in nature would have any evolutionary effect
in die teeth of even the slightest degree of adverse selection.
Pither mutation-rates many times higher than any as yet detected
must be sometimes operative, or else the observed results can be
far better accounted for by selection. A mutation with partial
dominance occurring once in lo® individuals will, if selectively
neutral, take a period of somewhat over lo® generations to estab-
lish itself in half the individuals of the species. If there were the
faintest adverse selection against it, it could never increase at all.
But if it conferred an advantage of only i per cent — i.c. if an
individual bearing one such mutant gene has an expectation of
reproducing itself which is only i per cent liighcr than those
without the mutant gene, then it would establish itself in half
the individuals of the species in a period of only about 10^ genera-
tions (R. A. Fisher, I930<i; Haldane, 1932a). Fisher (1937!') has
also made interesting studies on the form of the wave by which
advantageous genes advance.

Haldane (references and summary in appendix to Haldane,
1932a) has made a number of valuable theoretical studies on the
rate of evolutionary change to be expected with various given
degrees of selective advantage for autosomal dominants and
recessives and other types of mutations. One important conclusion
is that intense competition favours variable or plastic response to
the environment rather than high average response. This presum-
ably helps to explain the large variability to be found in many
natural populations.

For ordinary natural selection involving a simple dominant
with a selective advantage of i in 1,000 (i.c. where the ratio of
dominant to recessive changes from i to i ’Oor in each genera-
tion) it will take nearly 5,000 generations to increase the pro-

; AND BVOLtJTlON 57

portion of the domitiant from i to 50 per cent, and nearly 12,000
more to raise it to 99 per cent. For an advantage of i in 100 the

mimber of generations must be divided by lo. In the early
stages of selection of a single mutation with constant effects, when
the gene is still very rare, dominants can spread much more
rapidly than pure recessives, unless a certain degree of inbreeding
occurs.

These results may be actualized in certain cases: e.g. in dominant
melanism (p. 93) when conditions alter so as to favour the
melanic mutant, the rate of change in the constitution of the
population is of the order deduced. However, Haldane’s detailed
conclusions are not likely to be so directly applicable to evolu-
tionary problems as was thought at the time, since we now realize
that dominance or recessivity are themselves in large measure a
result of evolution, produced in response to the deleterious nature
of most mutations (p. 75). The mutations that are of value for
evolution will in most cases be of very small extent, of s%ht
effect, and often at least of incomplete dominance or recessiveness.
Further, we are now realizing that evolution vtill in general
proceed, not by the selection of single mutations, but by the
selection of mutations in relation to a favourable combination
of existing small gene-differences, or in many cases by the selection
of such new recombinations alone, to be followed later if occasion
offers by appropriate new mutations {p. 124). According to
R. A. Fisher, this process will be considerably quicker than that
of the selection of single recessives, which are the commonest
obvious mutations found, and accordingly were, when Haldane
wrote, usually considered to be the main source of evolutionary
variance.

It will be observed that the amount of variance provided by
mutation will, with a constant mutation-rate, vary directly with
the size of the population. In a given time, therefore, a rare
species cannot lay hands on the same store of mutations as would
be available to an abundant species. The problem of the relation
of size of population to evolution is, however, much more
complex than this (see p.200; R. A. Fisher, 1930a, Chap. 4, and
1937a; SewallWright, 1931, 1932, i94o).We must consider how

;j8 EVOLtfTlON: THE MODERN SYNTHESIS

much variance a population can hold, as weU as how much

variance it is provided with by mutation.

Many rare mutations must be extinguished by mere randon
loss: the individuals or gametes contaimng tliem fa to '

There must always be a tendency for mmonty genes,
are present in low frequencies, to be lost from &e
by such accidental extinction if of no selective advantage,
with a definite selective advantage such as i per cent, whic is
the order of magnitude for rapid evolutionary change, the
chances are strongly against a lone mutation
species (see, e.g.,- Haldane, 1939^’). Mutauons with a
efect will of course be lost through selection, the rate of

depending on the intensity of the effect.

Thus repeated mutation (i.e. a definite mutation-rate) together
with a considerable-sized population, are necessa^ tor new
mutations to have an evolutionary chance. Such abundant
species as have been analysed prove to be carrying a surpnsmg y
large number of recessive mutations in their germ-plasm ^see

p. 75, and Dobzhansky, 1937. Chap, s)-

In addition, the probability of mere accident playing a part m
the actual survival of particular genes or gene-cornbmations is
enhanced in small populations. This has been especially empha-
sized by Wright, who points out that we should expect to see,
in the case of small species or isolated subspecies, certain types ot
useless or even deleterious change, which would not occur m an
abundant form, becoming incorporated in the constitution
through chance recombination.

Already in 1912 Lloyd had discovered instances of this process
of accidental multipHcation and decline of mutant genes, but
without realizing its full theoretical impHcations; and by
Muller had drawn general attention to its importance. Scwali
* R A Fisher (1937a) points out that the number of the tare approximately
neutrai genes carried by a species increases roughly as the

lation-sle. Such genes, however, will only cause observable variability of my
magnitude if they can increase their as wiU occur if *^7

favourable or if changed conditions cause them to become so. Thus, as raid
{1940C. p. 89) points out, incicased variability ascribable to large population-s^
depend on genes actually engaged in causing evolutiomry chmge, and the
observed fact of such increased variability demonstrates the spread m nature ot

genes with small advantageous cflfects.

MBNDEUSM AND EVOLUTION 59

Wright later christened the process “drift”, and worked out its
consequences in full detail (see Wright, 1940).

Wliat may be regarded as the converse of the Sewall Wright
phenomenon of drift in small populations is the impossibility of
securing good results in artificial selection when only small
numbers are employed. This, the general experience of poultry-
breeders, has been confirmed by Hays (1940) in definite experi-
ments designed to test the point. Using a flock never exceeding
50 birds, and often much smaller, he was unable in the course of
eight generations to raise egg-production, though marked pro-
gress can be obtained by using large flocks. Apparently the
numerous genes needed for the requisite multiple gene-combina-
tions are not available in such small populations. In some charac-
ters involved in fecundity, indeed, the eflfect was contrary to the
direction of selection — a result comparable with die deleterious
changes sometimes seen in small populations in nature (p. 201).

The smaller the size of a natural population and the more
perfecdy it is isolated the more likely is drift to proceed to its
limit, resulting either in the complete loss of a mutation from
the group, or its fixation in all the individuals of the group —
accompanied, of course, by the complete loss of its normal aflele.

In larger and less isolated populations, however, drift wfll
normally proceed only within limits, causiog the frequency of a
gene to fluctuate round a position of equilibrium. This equi-
librium-frequency will be determined by the balance between the
two opposing processes of mutation-frequency on the one hand
and adverse selection on the other, while, as we have seen,
population-size will also have an eflect. Thus in large populations,
shghtly deleterious mutations may be present with reasonable
frequency, especially when recessive, and will then constitute a
reservoir of potential evolutionary change, since their unfavour-
able effects can generally be neutralized by appropriate combina-
tions of modifiers (pp. 68 seq.).

In some cases, as with haemophilia and other sex-linked
recessives in man, we know that the efect of a mutant gene is so
deleterious that a comparatively high mutation-pressure must
be postulated to account for its frequency. In other cases, changes

6 o

EVOLUtlON: THE MODERN SYNTHESIS

in external environment will alter the amount or even si^

t wt^ Seta o{ oTphnia

^tilnS on p. 52 , while the lowering of

the gladal period has doubtless led to autopolyplo <k replacmg

dioloids at high latitudes in. many plant species (p. 337)-

"'Se ^ be ™ of du„g« in

Genes may have their expression altered by rnodifiers

entirely to change their selective value (pp. 68 se^)-

Meanwhile it is important to re^ that ^utSon-

mutant genes represents an eqmhbrium between
freauencf and selection, that variability
eqi^biSm between recombination and selection and t^t

and structure of the population will have effects on both

'^WelSSvert later to this last ^ejewe "^W^ht

mention the important conclusion estabhshed by SewaU Wnght
(see Wright, I940u), that the greatest amount of evolutic^^y
potentialiW is available to large species divided mto parti^y
discontin^us groups (subsides etc.). The partial isoktion
between the groups favours diversity by local

by drift and the establishment of non-adaptive recombmations,

wUe the fact that it is only partial impUes that the varma
provided by all the diversity taken together is potentiaUy available

to the species as a whole. r j-

Recent work has emphasized the importance of studies
population-structure for understanding the precise way m which
evolution will operate in any particular species. Thus to take but
two examples, Dobzhansky ( 1941 ) points out Aat certam theo-
retical calculations as to the relation between the mutatton-rate
and the number of lethals actually found in a population will only
hold in unlimited populations. As the size of die normally in-
breeding population is deaeased, the number of lethals goes up.
In Drosophila pseudoobscura, using this method, he was able to
show that the size of inbreeding population-groups was quite
different in California and in Central America. A region with
size of populationrgroups will show greater divergence

MENDELISM AND EVOLUTION

between its constituent populations and these will each show
greater variability in time; further, in such a region, the type
as a whole can only change through the migration and selection
of superior genotypes from small colonies. In general, the size
of the constituent population-groups seems to be astonishingly
small for an organism with such capacities for distribution (see
also pp. 37 ’-2).

In Drosophila hydei the situation is rather different (Spencer,
1941). This is a tropical species which has become widely estab-
lished in U.S.A. as a hanger-on of urban man. Each dty and town
is the focus of a single population-group. Each such population-
group passes through tremendous fluctuations in size, becoming
quite small in winter. It is improbable that genetic equilibrium is
ever reached within such markedly fluctuating groups, and the
rapid increases in numbers give abundant opportunity for the
spread of new genes even against selection-pressure. Analysis
showed, as was expected, that different populations differed
markedly in the type and number of mutant genes that they
contained.

As Dobzhansky points out, wc may say on the basis of such
analysis that one of the most important recent evolutionary events
has been the merging in the human species of small population-
groups in a more or less freely interbreeding whole.

In general it seems clear that from the standpoint of mathe-
matical theory, existing mutation-rates will in moderately
abundant Species suffice, with the aid oi selection, tor the dis-
tinctly slow processes of evolutionary change to be observed in
fossils.*

This statement is a deductive one made on theoretical grounds
from the standpoint of mathematical analysis. In the remainder
of this chapter we shall deal with more concrete aspects of the
relation between Mendelism and evolution.

* In a stock like that of the horses, wtiich shows a functional evolution that
by geological standards must be called rapid, the time needed to effect a change
of specific magnitude is of the order of 100,000 generations, and to effect one of
generic magnitude of the order of 1,000,000 generations (Wells, Huxley and
Wells, 1930, Book 4, ch. 8).

evolution: the modern synthesis

2. GENES AND CHARACTERS

A great deal of water has flowed under the bridges of biology
sinre the early days of mendeUan work, when mendelim factors
were rigidly equated with mendelking characten when o^y
two states of a given gene were recognized, the dommant being
supposed to represent its presence and the recessive its absence,
and when all mutations and all mendelian genes VKie supposed
to have considerable and odiously discontinuom effects.

To-day the notion of mendelian characters has been
dropped (see, for instance, Sinnott and Dunn, 193A, p. 30i)- T e
term may occasionally serve as a usefiil piece of shorthmd
notation, but is in point of fact a false conception. In the first
place, a single gene may affect a number of characters, a phenome-
non known as plciotropism. Griineberg (1938). in ^ illur^atog
analysis, points out that pleiotropic effects may be reahzed m
three different ways. In the first place, a gene may exert a direct
cfiect on two or more distinct processes. The example o e
effect of the series of white eye-colour allelomorphs in Drosophila^
which also exert an effect on the shape of the spermatheca, is
probably an example of this category. Secondly, a gene may
exert a direct effect on a single process, but in many different
sites and conditions. This holds for the primary action of the
gene studied by Griineberg {1938) iu Ac rat, which ca^s
hyperplasia and abnormal growA of cartilage in Ae ribs,
trachea, and elsewhere. AnoAer and even more striking case is
the array of anomalies in such different organs as eyes and fet,
found in a particular strain of mice, whiA Bonnevie (1934) has
shown arc due to alteration in a single developmental mechanism,
namely the causing of embryonic blehs of fluid at a. particular
stage of embryonic development. _

The most interesting examples for our purpose, however,
belong to the third category, of indirect effects. A gene exerts
a primary direct effect, and this then causes numerous secondary

* A valuable summary of the modem outlook, which treaK cettain aspects
of the problem more fully than is possible iij -a single chapter, is given by ror

in bis little book, MendeHs0 and Et/oiution (li?34)*

MENDILISM AN 0 EVOLUTION 63

effects. Griineberg’s gene in the rat has a hyperplastic anomaly of
cartilage as its primary effect. But among the secondary effects
are such varied “characters” as emphysema, hypertrophy of the
right ventricle, blocked nostrils, and incompletely occluded
incisor teeth.'*'

An equally good example is that of the frizzled breed of fowl
(p. 118). Here the primary effect is entirely on the feathers; as
the secondary effect of the resultant abnormal heat loss, we find
{in temperate climates) marked thyroid and adrenal enlargement,
subnormal body-temperature, and much increased food-intake.

Such^^ondary effects are excellent examples of what Darwin
called “correlated characters”, which may be of great evo-
lutionary importance (pp. 188, 206, 533).

Furthermore, any given character represents the end-result of
a great number of genes interacting with the environment during
development, and is not inherited as such. What is investigated
in any genetic experiment is the inherited basis for a constant
character-diference. Thus a character-difference may be said to
be inherited in mendclian fashion, while the character cannot but
even so the differential effect of a particular gene on the character
need not by any means always be the same. It may alter according
to differences in the environment, and also according to differ-
ences in the remainder of the gene-complex. As an example of
the first, we may take the well worked out case of “abnormal
abdomen” in Drosophila (T. H. Morgan, 1915). This effect
depends on a single partially dominant sex-linked gene: but it is
ordy mani&sted in moist conditions. In dry conditions flies pure
for the gene appear perfectly normal, while intermediates are
produced by varying degrees of moisture (see Gordon and Sang,
1941, on the similar case of antennaless).

An equally striking botanical case is that of a type of albinism
in barley (Collins, 1927), dependent on a single gene. When
grown below 6*5° C. the plants entirely lack chlorophyll, while

* Waddington (19410) points out that cettaia ontogenetic events act as “epi-
genetic crises”, in that quite slight modirications of their coune will have a
considerable effect on a number of characten. Thus alterations in the pupal
contraction of Drosophila ate involved in mutant chaxaaeis of legs, wings,
bristles, etc.

64 EVOtUTION; THE MODERN SYNTHESIS

Aove 18" c. are quite

mutauB produce a graded amount of cUorophyU.

case (1923) in Printuk sinensis has now been shown to be due to

faulty experimentation. For other plant cases see Lawrence and

PricJf 1940). In Himalayan rabbits and Siamese cats (both simp

recessbes) black pigment is produced only below a certam

Sid temperami Normally only the extremities fall below

this; but njin (1927. 1930) has expemnent^y

extremities, and black on the body. AnoAer

is short-wing, a sex-linked re'cessive which at 27.5 C. markedly

reduces uung-leng* end a&cB

effect falls away with temperature and is absent at ^4, ,

1935). Thus environmental changes may either mask or bring
out L results of genetic difference. We must therefore disttii-
guish carefully between the nature of the gene and its expression.
The gene itself can only alter by mutation; but its expression can

be affected in a number of ways. ^

The most revolutionary change has come m regard to the way
in which the expression of a gene can be altered by other genes.
The discovery of this fact has given us the two fundamenta
concepts of genic balance and the gene-complex. Thm the internal
or genetic environment of a gene may produce effects upon its
expression which are as striking as those mduced by the exterm
enrollment, and of course very much more important from the
point of view of evolution.

By genic balance we imply that individual genes uct, not
absolutely, in virtue solely of di.eir inherent qualiti^, but re a-
tively, in virtue of their interaction with other genes. The concept
was first reached by studies on sex. It was at one time supposed
that in Drosophila and other forms with male heterogamety, one
X-chrornosome automadcaUy determined maleness, and two
femaleness. It has since been shown, however, that it is the
balance of the X-chromosomes to the autosomes (A) whi^ is
operative. A ratio of i X to 2 A produces maleness, of i X to
I A produces femaleness; wlide one of i X to about i - 5 A pro-
duces intersexuality; Sterile “super-males” and “super-females
are produced by ratios of i X to over 2 A and under i A

MENDEiISM AND EVOLUTION

respectively. Here we can deduce that sex-determination is
effected by the quantitative ratio between sets of male-determining
and female-determining genes, though we do not know how
many separate genes are involved in each set.

The principle, however, appears to be of universal application:
the effect produced by any gene depends on other genes with
which it happens to be co-operating. The effects of modif^g
genes are the most striking examples.

A classical case of the kind is the alteration in the hooded pattern
of rats by modifiers (Castle and Pincus, 1928). The basic gene
remains the same, but its effects may be reduced to a few specks
of black on the head, or progressively extended over the whole
back and most of the belly, by the agency of accessory genes.
In cotton (Gossypium) differences in leaf-shape have been evolved
in a precisely similar way (Silow, 1941)*

Extending this concept, we reach that of the gene-complex.
The environment of a gene must include many, perhaps all other
genes, in all the chromosomes. This gene-complex may be altered
in numerous ways by mutation or recombination so as to modify
the effects and mode of action of particular genes, whether well-
established ones or new mutations. We can thus distinguish
between the genetic and the somatic environment of genes.

Further, it is now known that a gene can exist in a great variety
of allelomorphic forms (alleles), up to a dozen or more being
known for single loci. The effects of these usually diflFer in a
quantitative way (though occasionally in a quahtative way as
well), and the steps between the various alleles may be very
slight. Multiple alleles are, in general, taken to represent different
states of a homologous material unit. They thus constitute one
type of gene-differences with quite small effects. Many modifiers
and cumulative factors such as those involved in quantitative
characters also have small effects. In many cases the actual origin
of such small differentials by mutation has been observed. Further,
where a gene’s effect is small, the variations of expression, due
to enviromnent and to other genes, may readily cause an overlap
with the phenotypic expression of an allele or of another gene
with similar type of expression. Thus though genetic variability

evolution: THE MODERN SYNTHESIS

must be discontinuous, its expression in measurable characters

may become continuous. i t - i

Even mutations which in one gene-complex are pathological,
in another may be perfectly harmless, and in yet another advan-
tageous. A striking example of this is provided by the work of
M Gordon (1931) on generic hybrids between the vmparous_
freshwater fishes Platypoecilus and Xiphophorus. Some strams of
the former possess a gene for the production of large pigment-
cells with black pigment, responsible for a certain type of spotted
pattern. When, however, this gene interacts with certain genes
in the sword-tail {Xiphophorus), the pigment-ceUs produce cancer-
like melanotic tumours (Kosswig, 1929)-
Equally striking and 'curious results may occur as the result
of the interaction of two gene-complexes in species-hybridization.
As an example of this, we may cite an intergeneric pheasant cross
recently described (Huxley, I 94 il>). The crosswas between a Lady
Amherst pheasant {Chrysolophus amherstiae) and an Impeyan
pheasant {Lophophorus tmpeyanus), and the hybrid was a tnale.
The coloration of the males of both parent species is not only
brilliant but varied. Thus the Lady Amherst cock has a black-
and-white extensible “cape” on the head, a striking regional
pattern on the body, and elaborately barred central tail feathers,
while the Impeyan cock is distinguished by brilliant patches of
burnished bronze, green, and blue-back on its upper parts, with
white rump and buff tail. The hybrid, however, has most of its
upper parts uniformly black, with mere traces of green and
bronze iridescence, but neither regional patterning nor briUiant
colouring. The lower parts and central tail feathers are mottled
with brown, grey and white in various ways. This simple colour-
scheme, by the way, cannot be considered to have any rever-
sionary or “atavistic” significance whatever: it is simply that the
dehcatcly balanced gene-systems responsible for the two elaborate
patterns have cancelled out, so to speak, to produce a wholly
different and almost uniform coloration. It may perhaps be
mentioned that in other characters the hybrid is intermediate
(c.g. the sliape and si:^ of the cape), and in still others shows
obvious dominance (c.g. the blue facc-skin of die Impeyan).

MENDELISM AND EVOLUTION 67

What wc may call partial genc-complexes may also arise in
relation to the separate chromosomes into which the total gene-
complex is divided. This has been demonstrated by Mather (1941)
as regards what he terms polygenic characters — i.c. quantitative
characters dependent on the co-operation and interaction of
numerous genes. In Drosophila, the number of ventral abdominal
hairs can be changed by selection so as to trangress the limits of
normal variability in both plus and nainus directions (cf. Casde’s
hooded rats, p. 65). But the effect of selection is exerted in two
main stages. During the first two generations, a marked change
is effected, wliich Mather interprets as being due to recombination
of whole chromosomes. Then, after a period of relative stability
for two or three further generations, a further and more marked
change is produced, continuing for a number of generations; this
appears to be due to recombination of originally linked genes
forming polygenic combinations for hair-number. Different
polygenic combinations for this character have arisen in homo-
logous chromosomes in different strains, each combination being
balanced in that it contains both plus and minus modifiers of the
character. Furthermore, a number of such combinations will tend
to co-exist in a species with considerable out-crossing, since the
delicacy of the balance (see below) is improved when the genes
for a polygenic character are heterozygous. When selection is
practised, crossing-over provides new and extreme combinations.
(Cf. the more fully isolated partial systems of Darlington; p. 362.)

No such balanced polygenic combinations can be detected in
the modifiers of abnormal mutant characters, such as bar eye.
Mather suggests that they will arise by natural selection in
respect of wild-type characters, in order to prevent too great
deviation from the normal, while at the same time affording the
possibility of change under selection, through crossing-over
providing a limited number of extreme recombinations. Such
polygenic combinations, like other features of genetic systems
(cf. p, 136), are thus a compromise between immediate individual
fitness and long-term evolutionary plasticity.

To sum up the evolutionary bearings of recent discoveries
about gene-complexes, we may say that evolution not only need

68 evolution: the modern synthesis

not occur by a »f ''“T ‘“P' *■“ “ ill™

sul ®p is immcfaely as it were, by auctlkry

1 ’ r-c in ffcnes and genc-combinations which can act as modi

S 'LTn^orjadng gene an! adjust i--

Xoh to the needs of the organism, though final adjustmen

nfav have to wait upon further mutatton. In any case what

\ i^ the eene-complcx; and it can do so m a senes of small,

if irJcgular ste^, so finely graded as to constitute a continuous

When we reflect further that it is theoretically possible for a
.el to Xr its character radicaUy by mutating step by small
Ttep from one member of a multiple allelomorph series to anoto
we^hall see that the discontinuity inherent in Mcndchan geneti
is no obstacle to the visible continuity revealed in palcontologic

evolution. Discontinuous germinal changes arc perfect y rapa

of producing continuous changes m somatic characters. N ,

Jc shall set forth more fully later, is the pathological character
of many mutations at their first appearance necessarily a bar

their final evolutionary utilization by the species. _

The divergence of two stocks will always involve tlj^jicei -
lation of different genes in the two Hnes, each bufferc
modifiers and adjusted in its own way to other genes; ^d tlm
will inevitably lead to a certain amomit of ^tsha^
aossfag, the Ft or kttr geucratiom bemg less
viiblc, or both (sec discussiou in Muller, 1939. Wo)- The
internal adaptation within the gene-complex thus automatically
helps to bring about the inter-sterility of species.

3 . THE alteration OF GENIC EXPRESSION
Let us take some examples of mutations, at first deleterious, being
rendered innocuous. One of the most striking cases occurs m the
meal-moth Ep/iesfiu kuhnklla. Here a red-eyed mutant i* known
which shows considerably lowered viabdity. But when the
recessive gene for red eyes is combined with another recessive
gene for transparency of eyes, the double recessive is as viable
as the normal wild type (Kuhn, 1934 )- < rn

A somewhat similar example comes (torn Drosophila. The

MENDEttSM AND EVOLUTION

mutation pt 4 rpk (eye-colour) causes the duration of life to be
considerably reduced. Another mutation, arc, affecting wing-shape,
produces nearly as great a reduction. But the two in combination
cause much less reduction than either separately (Gonzalez, 1923).
The figures (for both sexes together) are as follows:

Genes

Purple . .
Arc

Purple /arc
(Wild-type

Length of life (days)

24-54 ^ o-i8
26-81 ^ 0-29
33-71 ± 0-34
39-47 ±0 '^8)

Considering what severe effects are exerted by the two genes
separately, the favourable result of their combination is very
striking.* Brierley (1938) has worked out a means of determining
the “selective index” of any gene-combination, and has obtained
some suggestive prehminary results, also in Drosophila, on the
general viabihty interactions of numerous genes.

In a number of cases, the restoration of viability occurs gradu-
ally in the mere course of maintaining the mutant stock. The
classical analysis of this phenomenon is that of the eyeless mutant
of Drosophila.

Eyeless is due to a 4th-chromosome recessive gene. Its character-
istics on its first discovery were that it considerably reduced the
size of the eyes, in some cases to complete absence, decreased
fertility markedly, and had a depressing effea on viabdity. After,
however, a stock for eyeless had been inbred without any artificial
selection for a number of generations, it was found that practic-
ally all the flics had normal eyes and showed httle reduction in
either fertility or viability. On outcrossing to the normal wild
type and re-extracting the recessives in F2, it was found that these
once more manifested the original characters of eyeless, though
in even more variable degree (T. H. Morgan, 1926, 1929)-

The explanation of these facts is that the manifestations of
eyeless are readily influenced by other genes, and that in general

* As the stocks used in this experiment were not “isogenic** in regard to their
residual gcne-complex, an alternative explanation is possible, by which the
increased viability may have been due to modifiers and not to the specific
combination of the two main genes.

70 evolution; THE MODERN SYNTHESIS

those moaifiers which make for normal viabiHty and fertility
also make for normality in eye-size. Thus natural selection acting
upon the recombinations of modifiers present m the stock
speedily saw to it that the combination makuig for the mani-
festation of reduced eyes was eliminated. In competition wit i i s
wild-type allelomorph, eyeless would be eUmmated; but in stocks
pure S eyeless, the genes to be elimmated will be the plus
modifiers of the mutation. In broadest terms, there has been a
selection of the most favourable gene-complex.

A ^iniilar genetic modification of recessive rautattons towards
wild-type expression was found by W. W. MarshaU and Muller
(1917) in Drosophila melanogaster for the wmg-characters fcalioon
and curved. Still another example from Drosophila is the sex-
linked mutation vesicukted, affecting the wings; this can be
brought back to normal expression by means of autosom^
modifiers (Evang, 1925)- A very similar botanical case is recorded
by Harland (1932) for chlorophyll deficiency in cotton; in a way
this is even more striking, since the original manifestation of the
geiies (in this case three pairs are involved) was markedly Ictl^.
Here again inbreeding and selection led to the production of a
reasonably viable form, while outcrossing of this to normals
caused the reappearance of lethal segregants. The genetic mechm-
ism is timilar to that operative in the clastical case of Castle s
hooded rats and the alteration of their pattern m either plus or
minus direction by an accumulation of modifiers (Castle and
Pincus, 1928), though the selective impUcations are of course

different. 1 • j j

Selection of this type, it now appears, is a constant and indeed
normal process. It has become almost a commonplace in animals
used for genetic analysis to find that mutant types, which at first
are extremely difficult to keep going, after a few generations
become quite viable. This has repeatedly occurred in Gammarus,
for instance (Sexton, Clark and Spooner, 1930), and Mr. E. B.
Ford tells me that it has often occurred in his cultures of other
mutant stiains of the same species. A recently-described example
from Drosophila is that of white eye in D. obscura (Crew and
Lamy, 1932). This recessive mutant was at first very delicate

MBNDELISM AND EVOLUTION

but its viability improved progressively on inbreeding. A precisely
similar course of events was observed in the hairless mutant of
mice (Crew and Mirskaia, 1931), showing that the phenomenon
occurs in mammals. In plants, we have referred to cotton: an
analogous case has been found in the nasturtium (Weiss, quoted
by R. A. Fisher, 1931, p. 350).

In all cases, the explanation is basically similar to that for
eyeless. The experimenter, however, will also attempt to keep
the mutant character sharp: he will therefore be selecting for
combinations which keep the viability up without altering the
visible expression of the gene, so that the process may take a
little longer.^ R. L. Berg (1941) finds that, owing presumably
to this form of selection, dominance becomes more intense in
laboratory stocks than in the wild (see p. 75 seq.).

A converse effect is found when a gene, which in one species
or variety is harmless, becomes deleterious on outcrossing. The
explanation is that the expression of the gene in its normal
situation has become so conditioned by favourable modifiers
that it exerts no ill-cffects; on outcrossing, however, it finds
itself in a genic environment lacking some or all of these modifiers,
and consequently expresses itself in ways unfavourable to
viability. We have mentioned a case of this sort in Gordon's
fish crosses (p. 66). A stilking example, particularly relevant to
our present discussion, comes from Stockard's work (193 ^ 94 ^)
on dogs. The St. Bernard breed shows various symptoms of
hyperpituitarism that simulate the pathological condition known
as acfomegaly. Matings between St. Bernards give normal
litters; but when the St. Bernard is crossed with the Great Dane,
a breed that may be regarded as a simple giant type with no
hyperpituitary characters, a considerable proportion of the Fi

* Another method by which viability may be improved is by mutation in the
primary gene concerned. As an example, we may take the work of Mohr (i93^)*
Two stocks of vestigial-winged fruit-flies {Drosophila melanogaster) were main-
tained for a long time as inbred cultures. Iiv both of them, the wings eventuaUy
became almost normal. Analysis showed that this was due to the selection of a
less extreme allele of vestigial which had presumably arisen by mutation from
full vestigial, and had then been favoured by selection because of its less extreme
effects. An interesting point is that these nearly normal alleles were not identical
in the two cases, biit represented different steps in the multiple jeries.

72 EVOtUTION: THE MODERN SYNTHESIS

(and later) offspring show serious disturbances during their
growth, notably hydrocephalus and paralysis of the hind limbs,
these effects being clearly of genetic origin.

Man, it seems, has pushed the St. Bernard breed as far as it can
go in the direction of large size, heavy jowl, and other effects of
PYfreme or one-sided pituitary action; and in the process has
amassed those combinations of modifiers which will protect the
organism against the harmful effects of its exaggerated glandular
development. When the breed is outcrossed, the protective genes
are diluted to a greater or lesser extent, vdth corresponding id-
efltects. The modem show type of bulldog has similarly been
produced by selection for genes causing abnormal thyroid
structure and function; here the breed has been pushed still
further towards the glandular limit, since a considerable proportion
of males are partially or wholly sterile.

An example in which genic expression is altered without
noticeable effects on viability is that of the hybrids between
Drosophila melanogaster and D. simulans. About 50 per cent of
these lack bristles present in both parent species — ^i.e. certain
combinations of modifiers from the two parent species suppress
the expression of certain genes controlling bristle development
(Biddle, 1932).

Excellent examples involving artificial selection have resulted
from the work of R. A. Fisher (1935, 1938), who by repeated
back-crossing introduced dominant or semi-dominant genes from
domestic breeds of poultry, into the unselected gene-complex of
the wild jm:^le fowl. Polydactyly varies in its single-dose expres-
sion* in domestic breeds. Punnett and Pease (1929) found it to

The term dominant has unfortunately been employed very loosely, some
authors using it to -mean that the heterozygote is indistinguishable from the
homozygote, while others call a dominant any gene whose effects in single dose
can be distinguished at all: e.g. Bowater (1914) called the melaiiic form of the
moth Aplecia mhulosa a dominant, although the heterozygote, as he himself goes
on to state, has an intermediate expression; and all the so-called dominant muta-
tions in Drosophila melanogaster are either lethal in double dose, when, of course,
the visible effect of the homozygote cannot be determined, or their heterozygous
expression is less extreme than their homozygous (e.g. abnormal abdomen).

For this reason, and because it is in many ways unsatisfactory to have positive
and negative terms, like dominant and recessive, to denote gradations in what
is really a single scale of positive effects, I would suggest that some other term,

MENDELISM AND EVOLUTION

behave usually as a complete dominant, but sometimes as a
recessive and sometimes as an irregular partial dominant. To
explain these facts, they postulated one dominant inhibitor pre-
venting the expression of the gene for polydactyly, and a second
capable of suppressing the action of the first. Hutchinson (1931)
pointed out that a simpler explanation is provided by Fisher’s
theory, according to which there is a single gene which remains
constant, but whose dominance-relations differ in different gene-
complexes. In the wild gene-complex, moreover, as Fisher
showed, the homoaygote can be definitely distinguished from
the beterozygote by possessing larger extra toes, with more
bones. What has happened is that an originally intermediate
single-dose expression has, in most domestic breeds, become more
complete. The gene for barred plumage behaves in a somewhat
similar way. Most interesting are the results with the gene for
crest on the head. In the wild gene-complex the crested gene in
single dose produces crest alone; in double dose, however, it
produces not only an unusually large crest but a cerebral hernia
of deleterious character. In the Japanese silky fowl, no hernia is
ever produced, and the effect of the crested gene is the same in
single and in double dose. Thus firstly, the gene has become fully
dominant in domestication in place of intermediate; secondly,
its effect on hernia has been suppressed by modifying factors (cf.
pp. 70, 79) ; and thirdly, in the wild gene complex, its two effects
are of different type, the harmful hernia being fully recessive, the
neutral crest being partially dominant.

Another aspect of the adaptation of genic expression to the
needs of the organism concerns the stability of expression of
genes. Plunkett (1932), for instance, has analysed the fact, well
^own in general terms, that wild-type characters arc usually
much less modifiable by changes in environment than are those
determined by mutant genes. His analysis was for the most part

such as single-’dose expression (or heterozygous expresswity) would be more suitable.
Full single-dose expression would then be equivalent to true dominance; zero
siuglc-dosc expression to recessivity ; and truly intermediate expression to absence
of dominance in which the heterozygote is intermediate between the two homo-
zygotes. TimofeefT-Ressovsky (i934l>) deals with expressivity from a rather
different angle.

74 evolution: the modern synthesis

resKicted to temp^atute^fiects in ^ «l>e P™c|k

can be widely generalized. The explanation is based on the fact
that genes are in most cases concerned with the rates of proc^^s
(seeLldschmidt, m^a). The curves expressmg the rate of ^
processes are in general obhquely S-shaped, tendmg ^
Lrizontal equihbrium-position (Ford md Huxley 1929). la
wild-type genes, the flattening is normally completed before the
imaginal state (or corresponding defimtive stage) is reache ,
whereas in the majority of mutant genes, the curve b still m>e
ascending phase at this stage. Thus quite small disturbances of the
curve will have marked effects in mutant genes, but very shgfit
ones on wild-type characters.

There can be no doubt that selection has been at work to adjust
the rates of gene-controlled processes so as to produce to result
in wild-type genes, thus conferring a high degree of stabihty on
the characters concerned. As Plunkett further points out, the
evolution of complete dominance, with which our next section
deals, can be regarded as a special case of tliis principk, viz. that
in general natural selection favours the genotype which produces
the most stable and therefore uniform phenotype.

Where special circumstances demand the contrary effect, that
different conditions shall be met by quite distinct phenotypes,
selection has often operated to produce plasticity of genic expres-
sion. This plasticity, however, is usually of a special type, operating
by some sort of switch mechanism, so that two or a few con-
trasted phenotypes, each of them relatively stable, are produced.
The case is that of the environmental control of caste m

social hymehoptera, whereby the same genotype can be made to
produce either neuter or fertile females, and intermediates are
rare aberrations. The same sort of mechanism seems to be at
work in regard to the winged and wingless condition of aphids,
and in environmental sex-determination.*

* The same result can of course be secured geiytically, either by special
chromosomal mechanisms, as in genetic sex-detcrmination, or by a selcctiye
balance resulting in polymorphism, as especially well illustrated by butterflies
with polymorphic mimetic forms (sec pp. loi, 122).

MENDELISM AND EVOLUTION

4. THE EVOLUTION OF DOMINANCE

R. A.. Fisher (1928, 1931, 1934) hr s extended this concept of
the alterahility of gene-expression by modifiers to account for
dominance in general, or at least for many features of dominance
as found in nature. His argument runs as follows. Mutation is
always throwing up new genes; the majority of these will inevit-
ably be deleterious, since. in a delicately-adjusted system like the
gene-complex most changes are likely to be for the worse unless
compensated. Further, we know as an empirical fact that the
majority of mutations are repeatedly produced. Obvio'hsly the
great majority of mutant genes will be carried in single dose, so
t^t it will be an advantage to minimize any activity shown by
them while in the heterozygous state. Thus, even when a harmful
mutation at its first appearance shows considerable single-dose
expression, i.e. manifests some or all of its efiects when in the
heterozygous state, then, if it be repeatedly produced (which is
the case with most mutations), the way is open for it to' be forced
into recessivity by selection acting on the rest of the gene-complex.
If it is relatively abundant (and recent studies of void populations
— «g. C. Gordon (1936), Dubinin and others (1936), and Sexton
and Clark (i936fl) — ^have shown thfe surprisingly high incidence
of various recessives which they carry in the fly Drosophila and
the shrimp Gammams respectively), selection may get to work on
the homozygous condition, and render it also inactive. In such
a wayj as Fisher points out, mutations may be reduced fo the
rank of specific modifiers, normally inoperative, but exerting
effects in abnormal gene-situations.*

* As showing the intensity of the selection acting against certain recessive
mutations, C, Gordon {1935) Hberated 36,<X)oDro5<)|?/iilixmelii«()^<i5terin England,
where they are not endemic. The population .originally contained 50 per cent
of the recessive gene ebony (25 per cent pure wild-type flies, 25 per cent ebony,
and 50 per cent heterozygous for ebony). After 120 days (5 to 6 generations)
the frequency of the ebony gene had failen to ii per cent. From the data it
appears that some heterozygotes were selectively elimimted in each generation,
as weE as the homozygotes; tins is in accordance with the fact that ebony has a
slight single-dose expression.

A further important fact is that in nature recessives are almost wholly absent
from the X-chromosome of Drosophila, where, of course, they are exposed
(in males) to selection in single dose (p. 117).

Dubinin and others (1936) found more than one detectable mutant (recessive)

y6 evolution: THE MODERN SYNTHESIS

In support of tHs view, we will cite some of the array of facts
that show how readily the degree of doniinaiice of a gene may
be altered by the presence of other genes. The classical case is
that of horns in domestic sheep (Wood, 1905). The difference
between homed and hornless breeds depends on a smgle gene-
difference, but whereas a single dose of the gene for horns ^
produce horns in rams, a double dose is necessary m ewes. The
same gene is thus dominant in the internal environment of males,
but recessive in that of females. In fowls, Landauer (1937) foimd
that the gene for friz 23 ed plumage, whose effects are norm^y
incompletely dominant, is converted into an almost complete
recessive by the presence of a particular recessive modifier m
double dose. Dunn and Landauer (i 934 . 1936), with the gene
rumpless, which reduces the tail and posterior end of the body,
were able to go further and to show that this could be converted
either into a dominant or a recessive by crossing to different
stocks, followed by selection for dominance or recessivityjrespec-
tively. In mice, on the odier hand, the gene for black, which is
normally a complete recessive, can be converted into an incom-
plete dominant by modifiers (Barrows, 1934 )- hi Drosophila
milts, the dominant gene for rounded wings converts the gene
for ruffed bristles from a recessive into an incomplete dominant
(Lebedeff, 1933). Mather and North (1940) describe a gene in
mice whose only known efiect is to modify the dominance-
relations of the agouti gene.

"“A case of some historical interest is that described by Federley
(1911) of the behaviour of white spotting in the larvae of the
moth Pygaera. In P. anachoreta an unspotted variety is found, and
this behaves as a simple recessive to the normal spotted condition.
In P. curtula, however, only the unspotted condition exists. In
the Pi of a cross between the two spedes, using the spotted form
of P. anachoreta, spotting is expressed in an intermediate form,
i.e. its dominance has been partially abolished. Twenty-five years
ago, this fact seemed so remarkable that an authority such as

allele in each wild fruit-fly! They also showed that various mutant genes altered
in their frequency during three years, some becoming more and others less
frequent.

MENDELISM AND EVOLUTION , 77

Sturtevant (1912) was disinclined to accept it: to-day, however,
such alteration of genc-expression by modifiers has become a
commonplace.

We need not multiply examples. It is clear that the dominance
of a gene can be radically modified according to the genic
environment in which it happens to find itself.

The next step is to show that this undoubted fact of the modifi-
ability of the degree of dominance has been utilized in the course
of evolution to make most commonly-recurring mutations
recessive, so as to reduce the degree of their heterorygous expres-
sion, including that of the decreased viability which accompanies
most mutations at their first appearance.

In cotton, Harland (1933) and Hutchinson and Ghose (1937)
have studied the mutation crinkled dwarf. This occurs in the Sea
Island variety of Gossypitm barbadense, and is there a complete
recessive. When crossed to unrelated strains of the same species,
it is not completely recessive but shows a low degree of single-
dose expression.

When, however, the mutation was introduced from G.
barbadense into the related species G. hirsutum (upland cotton)
there proved to be a complete absence of dominance of the
normd type. The Fi is intermediate, so that at first sight we
might imagine the single-dose expression to be about 50 per cent;
but the fact that the F2 gives a large and unclassifiable range
shows that the degree of dominance must be under the influence
of a number of modifying genes. Tliis is confirmed by the results
of back-crossing the Fi species-hybrid bearing tlie crinkled dwarf
gene to various strains of G. hirsutum. In certain lines, complete
or almost complete recessivity of crinkled dwarf was re-cstab-
lished, while in othen the single-dose expressivity was rendered
accurately intermediate, the heterozygote class being clearly
separable from cither homozygote. As Hutchinson and Ghose
have clearly shown, the results entirely support R. A. Fishers
views. Later work (sec summary by Harland, 1941) has shown
that it also occurs as a very rare mutant in G. hirsutum. The
barbadense crinkled shows intermediate single-dose expression in
the Fi with hirsutum, but is recessive -when transferred to a pure

78 evolution: THE MODERN SYNTHESIS

hirsutum gcnc-complex. When, however, hirsutum crinkled is
transferred to a pure barbadense genc-complcx, it behaves as an

Harland (1941) develops die thesis that the dominance of
normal aUeles in cotton may be built up in ^ considerable variety
of ways. Thus the character petal spot in both barbadense an
hirsutum is based primarily on a main gene which, however,
exists in different alleHc forms in the two species. But whereas
in barbadense the action of this main gene must k remforced by
a number of plus modifiers to produce its foU effect m hmutum
the main gene is stronger, and requires no (or fc^) modifiers.
As corollaries of these facts we find (i) that m barbadense, the
modifiers exert some positive action (a smaU spot) even in the
absence of the main gene; {2) the gene (ox barbadense spot trans-
ferred to the hirsutum gcne^omplex has a very weak effect, and
the petals arc barely spotted; (3) the gene for hirsutum spot trans-
ferred to the barbadense gene-complex is remforced by the
modifiers there present, and produces a spot which is larger and
more intense than any previously known; (4) crosses of ±c
spotted forms of the one species with the unspotted of the other
give a graded F2 with all intensities of spotting; (s) the F2 troni
die unspotted forms of the same two species produces some spots
as large as normal barbadetise—i.c. due to recombination or
modifiers only, presumably from both species. In C. arhreum
yet a third method of producing the spotted character is found,
there is no main gene, but spotting depends on the genes that m
barbadense act as modifiers, but here must be called a multiple
factor series or polygenic combination.

Harland suggtsts that if a character is of advantage to the
species, it can be more rcaddy retained, in spite of recessive
mutation, if its expression depends on several genes. However,
this conclusion does not seem justified. The advantage of the
method of using a single main gene together with modifiers
would rather seem to he in keeping the expression of the character
relatively constant in the normal range of environmental cona-
tions, but retaining a considerable reserve of potential variabiUty
to meet new or extreme conditions. The singlc-^ene control will

MENDEUSM AND EVOLUTION 79

give greater stability, tbe purely multifactorial control (by
“modifiers” only) will give greater plasticity. Single-gene control
will permit the character to be more readily lost under the
influence of selection or by drift. Presumably in correlation with
this, in hirsutum, where spot depends on one gene only, the spot
character has been lost in all but a few rare varieties.

To return to crinkled dwarf, the same principles apply. Norm-
ality (non-crinkled) in barbadense depends on a single strong
allele, like spot in hirsutum. Normality in hirsutum, on the other
hand, depends on a weaker main non-crinkled allele, together
with a number of modifiers. These not only encourage the domin-
ance of normal over crinkled, but make the pure crinkled forms
more normal, both in appearance and viabihty. By rigorous
selection, new combinations of modifiers have been obtained
which render the hirsutum crinkled practically indistinguishable
from normal. When the two species are crossed, using the normal
of one and the crinkled form of the other, F2 ranges from forms
which are phenotypically normal through all grades of crinkling
to “super-crinkled” types which are almost lethal. The petal spot
experiments demonstrate two types of dominance. In hirsutum
the Fisher effect is clearly operative, with modifiers aiding a weak
“normal” gene, and also modifying the recessive towards
normality. Recessive modification is much harder where a nearly
dommant main gene exists, as in barbadense. This may then be
due to an extension of the Haldane effect (p. 82), by selection of
“stronger” normal alleles. Harland and Atteck (1941) consider
that this also operates for crinkled divatf in some species, but die
evidence is not decisive. They furdier point out tliat the Haldane
effect is likely to be more important in self-fertilized forms, where
the Fisher effect cannot so readily be produced (Haldane, I 939 ‘*)-
Where bodi types occur, as in Gossyp/MMi, doubtless “accidental”
events such as the time of occurrence of suitable mutations, will
determine which mechanism evolves (see also Silow, 1941)-

An important empirical fact which was among those that led
Fisher to promulgate liis theory is that of the behaviour of
multiple alleles. In almost every case so far investigated, the wild-
type ^cle shows complete dominance over ail the lower members

evolution: THE MODERN SYNTHESIS

of the series, whereas these when crossed with each other show
intermediate expression. Thus the normal red colour of the eye
in wild Drosophila is completely dominant over white, ivory,
eosin, cherry, and all the other members of the white allelomorphic
series: but white crossed with eosin, or ivory with cherry, gives
an Fi with eyes intermediate in colour between the two parents.

The exception proves the rule: and the exception to this rule,
to which attention was first drawn by Ford (1930), concerns the
efiect of this same series of genes upon an internal character of
apparently no selective value. Dobzhansky (1927) had shown that
the genes of the white-eye series affect the shape of the sperma-
theca to a small but constant degree. Whereas, as we have seen,
the genes are all recessive to wild-type as regards then: efiects on
the eye, they show intermediate expression as regards this second-
ary efiect on the spermatheca. The body-colour genes of the
ebony-sooty series show the same effect on the spermatheca, and
the same differential expression as regards their effect on the
external and the internal character (except that ebony body-
colour is not whoUy recessive). The most obvious explanation
is that selection has been operative in modifying the expression
of the disadvantageous external character, whereas no such effect
was called for, or has been produced, with regard to the harmless
secondary internal effect. We may also compare the different
dominance-relations of the deleterious and harmless effects of the
crest factor in fowls (p. 73).

It is worth noting that this differential expression of the gene
as regards two characters affected by it cannot be reconciled with
any rigid form of the “inactivation” theory of recessiveness. This
extension of the old Presence and Absence theory, which is
obviously untenable in its original form, claims that the degree
of recessmty corresponds to the degree of partial loss or inactiv-
ation suffered by the gene in mutating to one or other of its
recessive allelomorphs. It is dear from what we know of actual
deficiency-mutations, in which a portion of the chromosome is
missing, that loss may produce effects of the same nature as
gene-mutations (see, e.g., Mohr, 1920, who showed that that
loss of the white locus produces an ultra-white effect more intense

MENDELISM AND EVOLUTION 8l

than white itself), so that the inactivation theory may sometimes
apply. But the demonstration tliat some genes can become
dominant or recessive according to the gene-complex shows
that it can at best have a partial application. It is also noteworthy
that the true absence of a gene in ultra-white produces some
effects (spermatheca shape) which are hot recessive!

As R. A. Fisher (1931) points out, the majority of the characters
of most domestic breeds, or at least of the most obvious characters,
especially of pattern and colour, depend on mutations recessive
to the wild type. It would seem clear that man has here taken
advantage of two facts, first that more or less recessive mutations
are commoner in nature and secondly that they can be readily
fixed by mating two similar individuals, in order to utilize striking
and more or less recessive characters during his selection. Further-
more the natural tendency to concentrate for breeding purposes
on individuals showing a character in more extreme form will,
in tlie case of genes originally largely recessive, then automatically
encourage the production of complete recessivity. If, however,
die gene had more than intermediate single-dose expression,
selection would tend to make it more dominant. Thus in general
man’s artificial selection will tend to encourage either complete
dominance or complete recessivity, though for reasons quite
other than those operative in nature. But, as mentioned, the
distinguishing characters of domestic breeds in most species are
usually recessive.

The cliief exception occurs in poultry, where the majority of
“domestic” characters are partially or wholly dominant. Fisher
(1931) suggests that this difference is due to the fact that in
the countries of their origin, the domestic forms, even to-day and
more so in earlier times, would frequently be mated by wild
cocks, hi such a case, recessives could not readily be fixed, whereas
partial dominants would at least reappear in every generation;
thus dominants would tend to be bred into the race by a natural
selection of man’s selective processes. A further effect would be
that the degree of their dominance would be increased, through
the new varieties being almost wholly heterozygousrand through
man selecting the most striking individuals from which to breed.

82 evolution: THE MODERN SYNTHESIS

We have seen (p. 72) that his experimental tests have confirmed
this hypothesis.

Marchlewski (1941) has recently confirmed Fisher’s theory in
dogs. Here, black was originally dominant over yellow; how-
ever, in the dingo and in various domestic breeds, yellow has
become dominant through the selection of modifiers.

The reccssivity of characters in other domestic species is not
so nearly universal as Fisher was at first inclined to think (Castle,
1934, on various species; J. A. F. Roberts and White, I 930 > ^<1
J. A. F. Roberts, 1932, on sheep). It seems clear, however, that
man, by his breeding methods, has modified the single-dose
expression of wild-type genes in his domestic animals, accentuat-
ing the reccssivity of those with low, and the dominance of those
with high single-dose expression; and that while this will result
in most species in a preponderance of recessive breed-characters,
in poultry it will tend to a preponderance of dominant ones.

It should be stated that Wright (1934^) does not agree widi
Fisher’s views on the evolution of dominance, but wishes to
ascribe reccssivity to partial inactivation of the gene (p. 80).
An alternative hypothesis for the origin of the recessive character
of most mutations has been given by Haldane (1930; and see
1939‘j)- This is based on an interesting view as to the mode of
action of genes, namely, that different multiple allelomorphs
produce different amounts of some substance, but oiJy up to a
certain saturation value: beyond this they can produce no pheno-
typic effect. Thus any mutation in a minus direction below this
level can be detected, but those in a plus direction cannot. In
consequence a number of different multiple alleles of different
strength, but all above that needed to give the saturation value,
may readily accumulate without being phenotypically detected.
If now minus mutations occur, Haldane suggests that, in order
(I speak teleological shortliand) to prevent their visible and
viability effects from being manifested in the hetcrozygote, those
higher alleles will be selected which in combination with the
mutation will not fall below saturation level.

In other words, higher members of the series will be selected,
and visible dominance will be the result. Ford, however, has

MENDELISM AND EVOLUTION 83

pointed out that the saturation level itself will be deterniiried in
relation to the residual gene-complex, so that even here the
action postulated by R. A. Fisher may be operative, though in
some cases in addition to the Haldane mechanism. In any case
both suggestions involve selection acting upon other genes than
the mutant. Since the above was written. Ford {1940!)) has shown
that the Fisher effect can be artificially produced. The currant-
moth, Abraxas grossulariata, has a single-gene wild variety {lutea)
with yellow instead of white ground-colour, which normally
gives an intermediate Fi with wild type. By four generations of
plus and minus selection, Ford has conferred both complete
dominance and coniplete recessivity upon the gene.

Sewall Wright (1929, 1934a) attacks Fiber’s general conception
on the ground that the selection-pressure available will be
inadequate to achieve the results envisaged. However, there
seems little doubt that dominance of the “normal” wild-type
allele has been evolved; and Plunkett and Muller independently
(see Muller, 1935) have shown how the need for stability of gene-
expression in development will secondarily result in the evolution
of dominance.

Whatever the precise method employed, it seems clear that
dominance and recessiveness must be regarded as modifiable
characters, not as unalterable inherent properties of genes.
Domi n ant genes, or many of them, are not bom dominant; they
have dominance thrust upon them. Mutations may beconte
dominant or recessive, through the action of other genes in the
gene-complex. The evolution of dominance is thus seen to be in
large measure an adaptation to the deleterious nature of most
mutations.

R. L. Berg (1941) points out that the intensity of dominance
will be selectively bsilanced against the accumulation of deleterious
recessives which it makes possible, and tlrat it will tend to be
decreased in species consisting of incompletely isolated groups.
The extra plasticity thus conferred upon such species will be in
addition to that deduced by Wright (p. 229).

As another example of an adaptation of the genetic mechanism
itselfiFisher (1930a, p. 15) cites the plasticity conferred by sexuality.

84 evolution: THE MODERN SYNTHESIS

For one thing, it will permit evolutionary advance by the com-
bination of new mutations. If several favourable mutations occur
ill a population in a given time, then in a sexual cross-fertilizing
species they can be combined. But if the species is asexual, they
will almost certainly remain isolated, each confined to one line;
for them to be combined, one mutant must be selected until
it has become the main type, and only then will a second favour-
able mutation have a chance of becoming combined with the
first.

In the second place, it will permit recombination to throw up
new gene-combinations and so to use the existing genetic vari-
ance of the species to alter the type quickly in relation to changed
conditions. Thus it promotes both progressive specialization (see
Chapter 9) and plasticity in response to the clianges and chances
of the environment. In addition, as Fisher (1932) stresses, it has
a function to perform in relation to the deleterious nature of most
mutations. For, by allowing recombination, it permits mutations
to appear in homozygous form, and thus facihtates the ehmination
of the more deleterious. Elimination will be greater when the
frequency of homozygosis is increased by inbreeding or self-
fertilization. Thus variations in the type of sexual reproduction
will alter the emphasis of its evolutionary function (Darlington,
1939) : evolutionary plasticity will be more encouraged by cross-
breeding, evolutionary stability by inbreeding. Inbreeding will
also promote both the rejection of unfavourable and the spread
of favourable mutations (see also pp. 136, 140).

Recombinational plasticity vfiU be especially valuable when
conditions vary and become less favourable. Tliis is doubtless the
reason why so many organisms adopt some method of asexual
reproduction (which is more efficient reproduction) so long
as environmental conditions are favourable, but resort to a sexual
process as soon as they become unfavourable. This is so, for
instance, with many protozoa, rotifers, lower Crustacea, and
aphids.

The biological meaning of tliis has been clearly brought out
by careful genetic studies on Paramecium and other ciliates
(Jennings, 1929). It Iras been found in general that conjugation

MENDELISM AND EVOLUTION 85

causes an increase of genetic variability in the resultant population,
and that while (as in higher organisms) the majority of the new
U/pe may be regarded as unfavourable, some are actually or
potentially better-adapted than those prevailing before coiyu-
gation. Thus conjugation will in many cases provide an increased
chance of throwing up a recombination better able to cope with
unusual and unfavourable conditions. Precisely similar results
have been obtained with lower Crustacea by Santa and his
colleagues (see Davenport, 1933).

Attention is elsewhere drawn (p. 113) to the action of the
impulse to migrate in unfavourable conditions. This also confers
plasticity on a species, but in this case by increasing the range of
environmental opportunities available to a given hereditary
constitution, instead of increasing the range of hereditary consti-
tutions available to cope with given environmental conditions.
This, however, can hardly be called an adaptation of the genetic
mechanism.

On the other hand, the peculiar reactions of the crossing-
over mechanism to temperatures may well, as Mr. E. B. Ford
has suggested to me, fall into this category. In Drosophila, the
best-investigated case (Plough, 1917). crossing-over is least at
temperatures close to the optimum for the species, and increases
rapidly both with increase and with decrease of temperature.
Increased crosing-over will, of course, have the effect of increasing
the recombination of the genes located in a single kind of chromo-
some, and this will have a considerable effect in a form like
Drosophila where the chromosomes are few in number. Un-
favourable temperatures will thus increase the genetic variance
available to a population.

The discovery of the position-effect (pp. 48, 92) allows us
to deduce certain ways, previously quite unsuspected, in which
the evolutionary mechanism must itself have evolved. If, as now
seems established, it is the case with some or all genes that inter-
action with near neighbours in the same chromosome affects
their expression in an important way, then it is clear that all the
genes within a given chromosome must be dehcately adjusted' to
each other so as to produce a harmoniously functioning whole.

86 evolution: the modern synthesis

Aiiv ^ivcn gene must be adjusted to its neighbours witliin a
certain chromosome-distance either way; the genes at the limit
of tills range will be adjusted both to the gene at our hypo-
thetical starting-point and to others further away, thus conferring
a certain organization on the chromosome as a whole.

This involves a new conception of chromosomes. Up till quite
recently, it was possible and usual to regard them as mere vehicles
for the carriage and distribution of the hereditary constitution,
without any functional organization of the genes they contained.
These were assumed to be arranged at random, like coloured
beads picked up haphazard by a blind man and threaded on a
string; and tbeir positions in the chromosomes were not sup-
posed to have any relation with their effects on visible or other
characters.

With the discovery of the position-effect, however, this
asfumption, as a hard-and-fast principle, has gone by the board.
Although many genes affecting one character are scattered
irregularly through the chromosomes, and genes affecting
different characters are often contiguous, yet some degree of
non-random arrangement does occur (Morgan, Schultz and
Curry, 1940). Basically, and in origin, their arrangement doubtless
is random, and what we know of the frequency of sectional
rearrangements (pp. 90, 362) shows that genes must often change
their neighbours in an essentially accidental way. But dris random-
ness must then be given a functional polish: neighbouring genes
must be adjusted to each other by new mutation and by recom-
bination. To continue our metaphor, the blind man’s necklace is
looked at, and colour disharmonies are got rid of by choosing
new beads of the same general colour but shghtly different
shade.

The same general type of adaptation to position-effects has
been necessary as with dominance in relation to gene-mutation.
Indeed, the “weakening” of genes in abnormal positions (Dob-
zhansky, 1936, p. 376) indicates that a disproportionate fraction
of the single-dose expression of dominant genes is determined by
their relarions with their immediate neighbours. In addition,
functionally-balanced groups of genes affecting polygenic

MENDEllSM AND "EVOLUTION

characters will be evolved within the separate chromosomes
(Mather, 1941 : see p. 67). We must to-day consider chromosomes
not as being purely mechanical gene-vehicles, but to a certain
degree as organic gene-arrangements.

5. TYEES OF MUTATION

So far, under the head of mutation, we have been considering
only gene-mutation or point-mutation, i.e. the substantive
alteration of a definite unit-region of the chromosome outfit.
But various other kinds of mutation arc also known to occur
and we must devote a brief section to these.

In the first place, there arc genome-mutations, involving one or
more whole sets of chromosomes and therefore of gcnc-outfits.
The normal diploid complement of chromosomes of a species
may become doubled (autotetraploidy). Or reduction may fail
ta occur, and a diploid instead of a haploid gamete may be
formed. Or, as a result of the union of a normal gamete with
a diploid one, however formed, auto-triploid forms with three
genomes may result.

Tetraploidy in nature may also result from a cross between two
species, when the corresponding chromosomes from the two
parents do not pair before meiosis and the hybrid is therefore
originally sterile. If, however, the chromosomes of a cell divide
but not the cell-body itself, all descendants of this cell will be
tetraploid, and the two members of each kind of chromosome
can act as mates at reduction. The result will be that the gametes
have complete genomes from either parent. This is known as
allotetraploidy, and its actual origin has been observed in Primula
kewensiSy the fertile hybrid between P.Jiorihufida and P. uerticillata.
In this case the original hybrid was sterile, and the fertile type,
with larger leaves and flowers, arises sporadically in cuttings,
from a cell in which chromosome-doubling has occurred.
Allotetraploidy is almost confined to plants, because of the
favourable conditions provided by their vegetative growth for
the rare chromosome-doubling to occur and to give rise to
reproductive tissue, and because of their lack of the scx-cliromo-

88 evolution: the modern synthesis

some mechamsm normal to animals, which would not function
after chromosome-doubling (Muller, 1925; see also p. 14^)-
For polyploidy in animals see Vandel (i937)*

The presence of sk, eight, and more genomes in a strain^or
species is also known, again ahnost entirely in plants; presumably
the condition is usually consequent upon species-crossing (Chapter

6; and Darlington, 1937, P* 65 ). .

In all genome-mutations, the genome-units may be approxi-
mate only, sometimes with loss and sometimes with gain of one
or a few chromosomes (aneuploidy).

Triploid and other anisoploid strains with an odd number ot
genomes are relatively rare in nature, and cannot normally
reproduce themselves sexually, since the chromosomes of one
genome cannot find mates. But allopolyploids and other strain
with an even genome-number can maintain themselves. Such
polyploidy has undoubtedly been of considerable evolutionary
importance in plants. One method of species-formation is by
allotetraploidy after crossing (p. 34^)- apart from tHs,
polyploidy of any kind, so long as not excessive, by multiplying
the number of gene-pairs of each kind in the hereditary consti-
tution, confers long-range potential variabiHty and plasticity on
the species. For different gene-pairs may mutate in slightly differ-
ent ways, giving a gradation of new recombinations. High auto-
polyploidy, however, by virtually suppressing the chance of
manifestation of recessives, reduces plasticity. It appears that
the phenomenon has been of importance in the evolution of
higher plants, where scries of related forms with two, four, six,
and higher numbers of genomes often occur. Polyploidy has also
undoubtedly contributed to the evolution of many cultiyated
plants, notably the cereals and cotton. (See also pp.- 335 scq.).

The second type of chromosome-mutation is that of the
addition or subtraction of single chromosomes. This again appea.rs
to be much commoner in plants than in animals. It has been

^ Autopolyploids will originaliy produce many in viable gametes, owing to
the aggregation of chromosomes by fours instead of by pairs before mciosis.
But there is evidence to show that this condition, too, may be adjusted by selec-
tion, leading to reasonably true-breeding forms (Darlington, i937'» Muntzing,
1936).

^ ^ ^ AND EVOLUTION 89

thoroughly investigated in the Jimson weed, Datura stramonium,
by Blakeslee and his collaborators (1928). The commonest case
is where one ,kind of chromosome is represented three time-;
instead of twice in the hereditary outfit. Such fnso»i/c mutants, as
they are called, show greater differences from normal than do
ordinary gene-mutants. They also show reduced viabihty due to
the quantitative upset of gene-balance which they cause.

In Datura, these types cannot become fixed, since no pollen
with an extra chromosome is viable. Thus all viable poUen-grairu
will contain n chromosomes, while the ova will be either n or
w -b I .

It would seem, however, that in the evolution of some plants,
the condition has become stabilized; but this is always in polyploid
forms, where imbalance is not so readily brought about (p. 349).

The third type of chromosome-mutation is the sectional,
involving only parts of chromosomes. For its occurrence and
evolutionary bearings in Drosophila, see Muller (1940) . One form
of this is known as deficiency and involves the loss of a portion of
a chromosome. This is known to occur not uncommonly in
Drosophila, but is here and probably elsewhere of little evolu-
tionary significance, since homozygous deficiency is usually
lethal.

The converse is known as duplication, when a portion of a
chromosome comes to be repeated, occurring twice instead of
once, either in the form of a tramlocation to another chromo-
some, or of a “repeat” within the same chromosome, often
immediately adjacent to its original position. Small “repeats” of
this type have been shown by the sahvary gland technique
to be not infrequent in Drosophila, and are of considerable
evolutionary importance. They are of immediate importance,
since the alteration in genic balance would usually produce
definite but not deleterious effects. They are of much greater
ultimate importance, since they constitute the chief method by
which the number of genes is increased, thxis providing duplicate
factors, and the opportunity for slight divergent specialization of
homologous genes, giving great dehcacy of adjustment. In this
respect they would appear to be a good deal more important than

90 evolution ; THE MODERN SYNTHESIS

the earlier-detected and more spectacular process of duplicating
whole genomes by autopolyploidy (see pp. 334 seq.)-

The next type of sectional chromosome-mutation includes
all cases involving spatial rearrangement of sections of two kinds
of chromosomes. The most important form of this is reciprocal
translocation or segmental interchange. When this occurs, two
chromosomes break and exchange fragments. The precise mechan-
ism need not concern us. It can be induced with greater frequency
by X-rays and appears to occur where chromosomes actually or
almost touch each other. It is known to occur or can be deduced
to have occurred in a number of plants, and in Drosophila and

certain other animals. » i • i

In Datura, over forty so-called prime types , which dmer
from each other merely by rearrangement of segments of the
chromosomes, and which appear to owe their origin to reciprocal
translocations, are knovra to occur in nature; they do not show
visible differences. Different prime types differ in their geograph-
ical distribution. 1 1 • L

Owing to certain peculiarities of chromosome behaviour, these

prime types in Datura tend to persist as such, even after a cross.
This is in effect a form of isolation and should eventually give
opportunities for mutation and selection to produce visible
differences between the various chromosomal types.

After crossing between two prime types, the hybrid type will,
owing to certain chromosomal peculiarities, be reproduced as
such in later generations, without rearrangement due to crossing-
over, as well as the two pure types. If now a recessive lethal
mutation occurs in one of the chromosomes which have suffered
segmental interchange, the prime type containing that chromcn
some cannot be reconstituted, as a double dose of the lethal is
ex hypothesi fatal. Since lethals are relatively common types of
mutation, one may readily occur also in corresponding portions
of the chromosomes derived from the other prime type. If so,
and if we are dealing with an inbreeding group, we shall have a
condition of “balanced lethals”, and only the hybrid chromosome-
combination will be capable of survival.

Wherever much segmental interchange has occurred, followed

MEN0EI.ISM AND EVOLUTION 9 I

by long-continued intercrossing between the resultant prime
types, we may expect to find balanced-lethal and therefore
permanently hybrid combinations. And once these come into
existence, they can differentiate still further by the accumulation
of gene-mutations.

This, in actual fact, is what appears to have occurred in the
evening primroses, Oenothera (see Renner, 1925, and Cleland,
1928; and summary e.g. in Dobzhansky, 1937). One known
species (O. hookeri) is of normal cliromosomal behaviour, but all
the others present balanced-lethal chromosome combinations of
greater or lesser extent. The chief evolutionary significance of
these phenomena would appear to he in its providing a special
method of species-formation (see pp. 139, 329), It is, however, of
historical interest since occasional crossing-over will give apparent
mutations (really recombinations of large blocks of genes) ; and
on the basis of these de Vries advanced his mutation theory.

Small translocations of various types seem to occur quite
frequently in Drosophila. They have probably been of some,
though secondary, importance in initiating the differentiation of
species (Dobzhansky and Tan, 1936; and see p. 362).

As final form of sectioiul chromosome-mutation we have
inversion, in which one segment of a chromosome becomes
reversed within the chromosome as a whole. Quite large or very
small portions of the chromosome may become inverted. Here
again the frequency of tlje process may be accelerated by X-rays.

Crossing-over, of a type which will give viable offspring,
cannot occur in the inverted section of a chromosome paired with
a normal mate. This being so, inversion may produce a distinct
type, homozygous for the inverted chromosome, in addition to
the normal; in some cases, in fact, hybrids between the two types
will not be able to reproduce so freely, because of the death of
cross-overs. The resultant isolation of the two types of chromo-
some will permit their differentiation. More than that, selection
will tend to erect barriers to intercrossing, so that the resultant
waste due to the reduced fertility of the hybrids may be avoided.
In consequence, the two types may develop into distinct species.
This method of spcciation is discussed in Chapter 6 (p. 329).

gz evolution: the modern synthesis

We may here again mention the curious and mexpected
phenomenon of the position effect, according to which the mere
fact of rearrangement of genes produced by inversion, etc., may
cause a difference in their visible effects, thus simulating
(Dob2hansky, I93<5). Indeed, studies like those of MuUer (MuUer,
Prokofyeva, and Raffel, 1935) tn^ke it probable that a large
number of the genetic changes in Drosop/ii/u previously asmbed
to gene-mutation are in reality due to such “poation-eftects ,
produced by inversions of very small sections of a chromosome.

It has been suggested by some authors that what are normally
called gene-mutations are in reality only the effem of sma
rearrangements. However, Mackenzie and Muller (1940)
recently demonstrated that there is a real distinction between the
two types of mutation, since ultra-violet radiation can produce
true gene-mutation, but not the chromosome-breakage needed to
efiect sectional rearrangements, however small. This isprimajacte
evidence that the substantive changes due to true gene-mutation
do (as is to be expected) play a part in nature, in addition to the
organizational changes due to rearrangement of pre-existmg umts.

From the point of view of evolution, however, the significance
of such changes vnll bp very similar to that of true or substantive
mutation; the changes produced will be inherited according to
Mendelian laws, and will be of small extent.

Muller (1930) has also pointed out that if two homologous
chromosomes with different but overlapping inverted regions are
brought together by crossing, crossing-over will result in a new
type of chromosome containing one region in dupHcate. Such
small duplications will have visible effects, and may also, be em-
ployed as sources of evolutionary change. Recently the discovery
of the giant chromosomes in the salivary glands has converted
Drosophila from a very bad to by far the best material for detailed
chromosomal study, permitting the cytologist to produce a
detailed map of the visible structure of its chromosomes and to
detect even minute invenions and other rearrangements. It is as
if an astronomer armed only with Galileo’s telescope had been
suddenly equipped with a 50-inch reflector.

The use of this method has shown that rearrai^ements of

MENDELISM AND EVOLUTION

segments of the chromosomes are far commoner, and have played
a much larger part in evolutionary processes, such as the differ-
entiation of species, than was previously supposed. We have just
mentioned the important role of small inversions within a single
species. When we compare related species (e.g. Dobzhansky and
Tan, 1936; Dobzhansky, 1937), we find they are distinguished
by numerous characteristic differences in segmental arrangement.
Drosophila pseudoobscura and D. miranda, for instance, are so
closely related that they will mate and produce healthy (but
sterile) offspring. The chromosomes of the one are approximately
homologous with those of the other, but some segments have
been translocated to other chromosomes, and numerous segments
have been inverted, so that some sections of certain chromosomes
show “profoundly different patterns”. Other sections, however,
r emain approximately similar. On the other hand, it is probable
that such changes only pave the way for full separation, the later
stages of speciation being effected by a series of single gene-
mutations (see p. 359, and Muller, 1940).

In barley, however, Gustafsson (1941) fods that induced sectional
rearrangements occurring simultaneously with induced gene-
mutations are most likely to give favourable results, as providing a
new internal environment for the new gene (cf. pp. 67, 552).

But in spite of the frequency of these larger types of mutation,
it would seem that gene-mutation, together with the “pseudo-
mutation” due to position effects, is the most important source
of evolutionary change.

6. SPECIAL cases; melanism; polymorphism;

FLUCTUATING POPULATIONS

Before proceeding further in our main argument, however, we
must turn aside to discuss certain special cases which illustrate
various points concerning nco-mendehsm and selection.

{a) Melanism in moths

The first of these concerns the phenomenon of melanism in
moths, which has played a prominent part in recent evolutionary
discussions. The facts may be summarized as follows:

evolution: the modern synthesis

In the first place, there is no doubt that melanism among many
.species has become much more frequent during the last century,
and that this change has been associated with industrialization:
the predominance or abundance of melanics occurs in large cities
and in industrial areas.* In some cases the entire population of an
area has become melanic. Descriptions of the historical course of
events have been given for Britain by Harrison {1920b, 1932),
and for the continent by Walthcr (1927)- and by Hasebroek
(1934.). A summary of their genetic basis is given by Ford (i 937 )-

As regards their origin, Harrison (1928, 1935) claimed that he
had been able to cause melanic forms to appear, in two species
belonging to different genera, by means of incorporating lead
and manganese salts in the food, and that the induced melanism
behaved as a mendeHan recessive, as docs the naturally-ocairring
but very rare melanism of these species; (in a third species in
which natural melanism is dominant, and has shown industrial
spread, he abandoned the work after only prehminary results).

However, repetition of the work on a large scale by Hughes
(1932) and Thomsen and Lemcke (1933) has failed to confirm
these results. R. A. Fisher (i933<i) has also criticized Harrison’s
views as involving a mutation-rate much higher than any obtain-
able by X-ray treatment. Furdier, all industrial melanism is due
to dominant genes (see below). It would seem best to assume that
some error has been at work, and that no true induction occurred.
If so, then melanic mutations must, like other recurrent mutations,
have been thrown up sporadically for a long period, but have
spread owing to the altered selective conditions of an industrial
environment. Numerous cases of sporadic melanism which have
not become more frequent recently are known in moths as in

* It has been claimed by Harrison that melanism is also commoner in coastal
areas: Ford (1937), however, shows that this conclusion is certainly not universal.

Hardy (1937) states that slight but definite darkening has occurred in the
house-sparrow {Passer domesticus) in the Liverpool area. It will be of interest to
see whether this change, too, will show progressive spread. Sporadic melanism
has occurred in the passerine West Indian bird Coereha ftai>eolar pxod\icmg four
separate melanic island subspecies. In some cases the replacement of the normal
by the black form has been followed during recent historic times. Furthermore,
almost all island forms of the genus are somewhat darker than the mainland
forms (Lowe, 1912). Two cases of recent spread of melanic mutants in mammals
are considered later (pp. 103, 104).

MENDELISM AND EVOLUTION

other groups. An interesting case is Boarmia extersaria. This
shows industrial melanism in Germany; but in Britain it does
not enter industrial areas, and the melanic type has remained
sporadic. A survey of industrial melanism reveals that the intensity
of darkening varies from species to species. As regards its genetic
basis, no case of recessive melanism is known to have shown
industrial spread. Industrial melanism always depends either on
a single dominant gene, or on multiple factors each exhibiting
complete or partial dominance (Ford, 1937).

As regards its physiological efiects, numerous authors have
shown that dominant or partially dominant melanism confers
extra hardiness and viability. Ford (i94oh) has shown in Boarmia
repandata that in highly unfavourable conditions (feeding on
alternate days) the ratio of melanics emerging (on an expectation
of 50 per cent) goes up from about 54 to about 70 per cent. He
has also (1937) pointed out that, in spite of this physiological
advantage, dominant melanic forms in non-industrial areas have
not shown any spread or increase in frequency.

He accordingly concludes that recessive melanism is due to
genes which have been forced to become recessive by selection
of modifiers, on account of their deleterious effects on viability.
Dominant^ melanism, on the other hand, has favourable effects
on viability, but in normal conditions is kept in check by counter-
selection operating through natural enemies, the type forms being
definitely cryptic in coloration, while the melanics stand out
sharply against the normal background. A balance is thus
reached, with a low percentage of melanics.

In industrial areas, however, the counter-selection in favour of
the type is not so strong, since the background is darker. It has
been suggested that m some cases ecological selection would here
be reversed, and the dark forms become better protected by
background resemblance. Detailed counts by Harrison, however,
have shown that in some species at least this is not the case. In
industrial areas it is further to be expected that many natural
enemies of the adult would be reduced in numbers or absent,
so that selection for concealment would be less stringent. As a stiH
further possibility, it appears probable that in the chemically

96 evolution: the modern synthesis'

unfavourable environment of industrialism, the greater hardiness
of nielanics would have an increased advantage.

In any case, according to Ford’s general hypothesis, the balmce
between opposed selective forces is in industrial areas tilted in
favour of the melanic variety, with the result that this has speedily
increased and in some cases has completely ousted the original
type. The strength of the selection acting gainst melamcs on
account of their coloration is shown by the fact that in '^ne
species, although the melanic is more cold-resistant, and has spread
in industrial areas,it is not found so far north as Ae normal.*

It would appear that on no other hypothesis can the lack o
spread of dominant melanics in raral areas and their spread in
industrial areas be reconciled. If so, we have one of the most
striking demonstrations of the efficacy of selection.

{h) Genetic polymorphism

Genetic polymorphism, or the co-existence of two ot mote
genetically-determined and well-defined forms ( phases ) o. a
species in the same area, presents certain pecuHar problems. We
speak of polymorphism when the difference between the various
forms is sharp, or at least expressed as a bi- or multi-modaUty in
a frequency curve of variabiHty; when the equihbrium between
them is relatively stable; and when the frequency of the least
abundant is high enough to make it certain that it is not due
merely to mutation-pressure (see Ford, 1940^). Polymorphism
must be clearly distinguished from normal variabiHty, however
extensive, which will be grouped in a single ummodal frequency
curve. The existence of separate forms or distinct modes is an

essential characteristic of polymorphism.

Since we are here concerned only with genetic polymorphism,
we can neglect all such cases as those of the social hymenoptera,
the seasonal forms of certain butterflies, etc., which are determined
environmentally. We can also neglect the particular type of

* In certain cases, the melanic form has spread from its original industrial
area into surrounding non-industrial country, with a decreasing frequency-
gradient. If the selective balance in favour of non-melamcs m non-mdustriai

areas is slight, this is to be expected as the result of mere population-pressure.

cf. Sumner’s views on population-pressure ill subspecies of PtTOttiyiTns (p. 187).

MENDBLISM AND EVOLUTION 97

genetic polymorphism involved in the genetic determination of
two sexes, since this is primarily maintained not by a selective
balance but by the inherent nature of the genetic-reproductive
mechanism; the same applies to heterostyly in plants. Paulian
(1936) applies the term genetic to certain cases of polymorphism
in male insects where the forms are discontinuous and not to be
explained by simple allometry (heterogony). Until, however,
the developmental basis of this phenomenon has been ascertained,
it is better not to assume that it must be genetically determined:
see also Huxley (1932, Ghap. 2, § 5).

The interest to the evolutionist of genetic polymorphism within
a freely interbreeding population is that, as R. A. Fisher (1930(1)
was the first to point out in general terms, it must always involve
a balance of selective advantages between the difierent types.
For, ex hypothesi, mutation-pressure alone will not account for
the facts, and it can readily he shown that in the absence of
selective balance, one type would rapidly oust the other from
any considerable representation in the population.

There are two distinct methods by which this balance is actually
efected— genetical and ecological.

(i) In the case of genetical balance, the heterozygote is more
viable or enjoys some other selective advantage over either of
the bomozygotes. (For simplicity’s sake we will consider only
cases involving dimorphism; trimorphism will occur, as in
certain species of foxes, when the heterozygote differs in appear-
ance frorn either homozygote, or in other cases, as in Papilio
polytes and P. memnon, when two interacting gene-pairs are
involved (Ford, 1937), and polymorphism when two or more
non-interacting gene-pairs are involved.) This may occur in two
ways. Either the gene itself is less viable or even lethal in homozy-
gous condition; or it is closely linked with a recessive lethal, which
exerts no effect in single dose but is lethal when homozygous.

Owing to the difficulty of proving a negative, no certain case
of the former conditions is known, though the mutant curly in
Drosophila melanogaster is a possible example. In the conditions
of ordinary laboratory cultures, this maintains itself indefiuitely,
giving a dimorphism with wild-type. It does this because it is

98 evolution: the modern synthesis

almost fully lethal when homozygous, but usually rather more
viable than wild-type when heteroz)^goiis. As, however, it is
located within an inversion, the possibility of linkage, either with
a lethal or with the genes for tlic increased viability, is not ex-
cluded. Of linkage with a lethal, however, two examples have
been thoroughly worked out. In the ease of the butterfly Argynnis
paphia, over most of its range an occasional second form of female
occurs, known as valesina, in which the ground colour is dull
green instead of rich brown. This form is usually rare, but in
some localities constitutes 5 to 15 per cent of the female popu-
lation. Goldschmidt and Fischer (1922) showed that valesina
depends on a dominant gene closely luiked with a recessive
lethal; they were able to break the linkage, thus obtaining broods
with ail females of the valesina type. In China and neighbouring
areas almost all the wild females are valesina. This well show^s the
relativity of the term normal as applied to organisms in nature.

A similar situation exists in the American Clouded Yellow
Butterfly, Colias philodice (Gerould, 1923), Here a white female
variant exists, and normally constitutes 4 to 20 per cent of wild
females. As witli ArgynniSy the rarer form is due to a dominant
gene linked with a recessive lethal, and Gerould was able to
separate the two genes. In one area the white type is the more
abundant. The situation as regards selective advantage is Com-
plex. The lethal must have some advantage (a) when hetero-
zygous, as it is (in Colias'j more widely spread in the population
than the dominant colour-gene; (i) in association with the
dominant colour-gene, since the linkage, though not very close,
survives in nature in most areas. The dominant colour-gene
must also have some advantage in association with the lethal
to balance the wastage arising from homozygosity. Where it is
homozygous in nature, the advantage must come from association
with some other gene.

Numerous cases, as yet unanalysed genetically, are known in
nature where polymorphism, as with Argynnis paphia, exists in
one area of the range of the species but not in another, A familiar
example is the common red squirrel, Sciurus vulgaris, wliich is
always red in some areas, e.g, Britain, but both red and black in

MENDELISM AND EVOLUTION

mountainous parts of Europe {Liihring, 1928). Other cases are
mentioned in Chapter 5 (pp. 184, 203). In some it would appear
tliat a new area has been colonized exclusively by individuals of
one type, which presumably then is not linked with a full lethal,
but eitlier is linked with a gene somewhat reducing viability,
or is the type which is less advantageous than the lethal-linked
form. In other cases the lethal linkage may have been broken, as
with Argymis and CoUas.

Highly polymorphic species exist in nature among land-snails,
such as the common Cepaea nemoralis and C. hortensis (A. Lang,
1908), with their vast range of ground-colour and degree of
banding; grasshoppers (grouse-locusts, etc., Nabours, 1925; grass-
hoppers, Rubtzov, 193 s*); and certain fresh-water fish sikIi as
Lebistes (Winge, 1927). In tliese cases the polymorphism appears to
depend on the phenomenon of close linkage within each, chromo-
some or of the obligatory association of many whole chromo-
somes to produce a similar effect to close linkage (Fisher, ig^ob;
Diver, 1932). In these circumstances, a recessive lethal will prevent
the free recombination of any favourable mutations in the same
chromosome. Thus, since recessive lethals are common types of
mutation, whole chromosomes will have to compete with each
other, instead of selection being able to act so as to produce an
approximation to a single “best” combination of genes. Further,
since homozygotes cannot live, there must be at least two different
for ms of each of the chromosomes which contain lethals; the
different combinations of these vdll of necessity give a variety
of forms; and this variety wiU itself be subjected to selection so
as to give the best balance, and the least waste through excess
mortality of one or some of the forms. In the land-snails, the
interesting fact thas been discovered that the frequency of the
different types of banding has remained about tlie same since the
neolithic period (Diver, 1929), showing that the balance is an
enduring one. The problem, of course, remains as to why the
close linkage has become a characteristic of the species in the
first instance, since no obvious advantage inheres in such a

* Parallel variations occur here as in but in much more striking

fashion, since they affect a large number of related species and indeed genera
(P* 516).

100 evolution; %HE MODERN SYNTHESIS

condition. Haldane (1930; and see discussion in Ford, 1934,
p. 84) has suggested that it is due to translocation of segments!
from one chromosome to another. The unusual phenomenon of
the dominance of the mutant types over a “universal recessive”
would then also be accounted for, since the mutants would possess
the translocated segment in duplicate, both in its original and its
new position. Further, if, as is to be expected and as appears
actually to be the case, the translocation is less viable when
homozygous, we could dispense with lethal genes as an explan-
ation of the selective balance reached.

The common sea-anemone Metridium senile exists in a number
ofstrikingly different colour-varieties. D. L. Fox and Pantin (1941)
enumerate eight, including white, red, brown, grey, and various
combinations of these; in addition, there is much variation in
the intensity of the colours. The difeent forms occur in different
proportions in different localities. The various colour-types
depend on the interrelation of (i) lipochrome, giving colours
from red to yellow; (2) brown melanin, restricted to the ecto-
derm; {3) black melanin, restricted to the endodem. There
seems no doubt that the main types are genetically determined,
and differ in their metabolic properties, and also that the colour
is here adaptively non-significant, but correlated with some basic
physiological difference. Fox and Pantin conclude that selection
is weak as between the different colour-varieties, and that this will
account for the existence of the numerous phases. We have seen,
however, that a selective balance is required for this, and experi-
ments on the physiological and ecological differences between the
varieties should yield interesting results. There are the additional
complications that asexual reproductions occurs, and that single
individuals can persist for great lengths of time, perhaps even
indefinitely.

In the Mexican fresh-water fish Platypoecilus maculatus, M.
Gordon {1939) has found well over 120 patterns in a state of
nature, mostly dependent on the recombination of 15 gene-pairs.
This is a remarkable degree of polymorphism for a wild species,
especially as collections dating from 1867 indicate that it is a
balanced one. Furthermore, as in the -^ther examples we have

MENDELISM AND EVOLUTION ' lOI

cited, related species and genera {Xiphophorus) present modi
parallel variation. Some of the excessive variation, however, is

apparetidy due to the fixation of “accidental” characters by drift
(see pp. 58, 128). A quite difierent type of balance occurs in
variable plant species which consist of numerous ecotypes (pp.
275.276).

(ii) Ecological balance, on the other hand, depends either on
a diminution in the degree of selective advantage due to increase
of frequency of one or all of the polymorphic forms above a
certain level, or on an alternation in the amount or type of
selection due to alteration in the environmental conditions. The
best examples of the former concern mimetic butterflies. Either
all the forms are mimetic, or one is non-mimetic and the other
or others mimetic. If a mimetic form happens to become too
abundant relative to its model, the protection afforded by the
resemblance will diminish. Where certain mimetic for ms are
wholly absent, the corresponding models are also found to be
missing. In any area, a balance will thus be struck, depending on
relative abundance of models, intensity of predation, and viability
factors (p. 191).

Among the best-analysed examples are those of Papilio polytes
and HypoUmnas dubius. In the former case, only the females are
polymorphic, existiiig in three forms, two mimetic and one
non-mimetic. The species is a successful one, able to live outside
the range of its models : it is then, of course, monomorphic, aU
females being non-mimetic.

It seems clear that where models are available, mimicry confers
a definite advantage, but one which diminishes rapidly with
increased frequency of the mimetic forms.

In HypoUmnas dubius, both sexes are alike, and are dimorphic,
with two mimetic forms (see pp. 123-4) . For the details of the
genetic basis of the condition, readers are referred to Ford (1937).
In general the equilibrium due to ecological selective balance may
be broadly compared with the effects of mass action in chemistry.

As a matter of fact, Papilio polytes appears to illustrate a combin-
ation of genetic with ecological control, since the two mimetic

102 evolution: the modekn synthesis

forms, both of which arc dominant, are more viable in the
heterozygous than in the homozygous condition.

Various authors have regarded as a theoretical difficulty the
fact that such enormous differences in pattern, and even shape
and habit, obviously involving many independent characters,
can be controlled by a single gene. It should, however, be clear
that once a mutant type is established, conferring even a small
mimetic advantage, the residual gene-complex can undergo
evolution of the same nature as what we have discussed in
previous sections (pp. 98 seq.). But here the extent of such modifi-
. cation may be pushed much further, with a result diat is best
described, not as a modification of the visible effects of die
original gene, but as an addition of various new effects, all of
which, however, arc dependent for their expression on the
presence of the original gene. Presumably tliis change in the gene-
complex will depend more on new mutation and relatively less
on recombination of previously existing genes than in e.g.
alteration of dominance (see Ford, 1937).

We may put it in another way by saying that the original
gene-difference comes to act as a switch controlling the action
of numerous mutant modifying and modifiable genes, precisely
as occurs in the case of the primary sex-difference in regard to
genetically sex-limited characters. There is no reason for, and
every reason against, postulating the sudden origin of the whole
pattern by one mutation. Further, the frequent superficiality of
the characters by which mimetic resemblance is achieved shows
that the resemblance cannot have arisen through parallel muta-
tions occurrii^ and being preserved owing to similarity of
conditions (see Punnett, 1915 ; Cott, 1940, p. 405). The apparently
cryptic colour-polymorpliism of certain stick-insects and mantids
may have a similar ecological basis. The egg-colour polymor-
phism of the cuckoo, Cucuhs canorus, h largely related to the
risk of ejection by the host Qourdain, 1925; Huskins, 1934).
The different egg-colour strains are presumably balanced in
relation to host-abundance.

In the ruff {Machetes pugnax) polymorphism is confined to
males in the breeding season, and the number of distinctive

MENDELISM AND EVOLUTION IO3

types is extraordinary. No other bird rivals it. Ford (1940CI,
p. 501) suggests that the cumulative effect on females of display-
stimulation by numerous males at a common courting-ground,
(p. 480 Darling, I939; Huxley, 19380) promotes ma.ximum vari-
ability. Mayr and Rand (1937) cite a peculiar and striking dimor-
phism in taU-coloration in the bird Rhipidura brachyrhyncha.

Polymorphism is much commoner than usually realized,
e.g. in birds and mammals only, it exists among squirrels, foxes,
bears, cats, owls, herons, hawks, skuas, etc.

Next we come to cases where chaises in external environment
determine the ecological balance, not those in internal conditions
(relative abundance of the separate types). We have mentioned
the common squirrel (p. 99. For plant ecotypes, see p. 275).

Elton (quoted by Ford, 1934, p. 83) has found that in the red
fox (Vulpes fulva), which shows trimorphism, the “cross” type
being apparently a heterozygote between red and silver, the rare
silver type changes in its relative frequency in a regular way
during each of the lo-year cycles of abundance to which the
species is exposed (p. iii). In this species. Cross (1941) finds a
rough polymorph-ratio dine (p. 222), red being commoner to
the S., silver and cross to the N. (and see p. 185).

The arctic fox {Alopex lagopus) is dimorphic in winter (blue
or wliite). Only the white type occurs in Kamchatka, and only
the blue in certain Alaskan islands; while on the Alaskan main-
land a N-S gradient is found, the white type decreasing in fre-
quency with latitude. (See also p. 217.)

Here any selective advantage afforded by the white coat in
winter must presumably be offset by some disadvantage, probably
connected with viabihty, for the blue type to be able to exist in
numbers at all. The primary basis of the dimorphism (as in the
red fox) is thus a genetic balance. But environment may somewhat
alter this equilibrium, so that the balance is in part also ecological.

A remarkable case where the relation with environmental
conditions is more direct is that of the hamster (Cricetus erketus),
as described by Kirikov (1934) and Timofeeff-Ressovsky (i940j-
About 150 years ago the naturalist-geographer Lepekhin noted
that in a certain region of Russia black hamsters were unusuaEy

104 evolution: the modern synthesis

prevalent. Since then, the statistics of the Russian fur-markets
have enabled biologists to trace the steady spread of the black
type, until to-day, tliroughout a broad zone along the northern
border of the range of the species, black forms are in the majority
and in some areas are present to the total or virtual exclusion of
the typical greys. This area coincides with the sub-steppe (wood-
steppe) climatic zone, and is cooler and moister than the steppe
region proper, which forms the main home of the species. In the
steppes the black type occurs only as an occasional aberration. It
seems clear that the black form enjoys some selective advantage
in the sub-steppe area, while it is at a disadvantage in the drier
steppes.

In the bird Coereha, the recent replacement of the typical by
the mclanic form on certain West Indian islands appears to be
similar (pp. 9411, 203). In other cases, although the actual process
of spread has not been followed, we can be certain that it is
taking place. For instance, in the brush opossum {Trichosurus
vulpecula), melanic variants are very rare on the AustraUan main-
land. In the Tasmanian subspecies, however, they are common,
Pearson (1938), from an examination of many thousand skins,
was able to plot a contour (“phenocontour”) map of the relative
frequency of the black type, with contour lines (“isophenes”)
marking regions of a given frequency. In the first place, he was
able to show tliat neither an isolated small island off the north-
west, nor another on the north-east on the course of the sub-
merged land-bridge from Australia, contained black animals
except as aberrations; chis demonstrates that the abundance of
the black type must have arisen after the isolation of the sub-
species m Tasmania. The north-west comer of Tasmania contains
only black opossums, while on parts of the east coast the pro-
portion is under 25 per cent. There is no correlation of the
frequency of blacks with climatic gradients. The suggestion that
the black type appeared (whether by new mutation or by the
crossing of two carriers of the black gene in single dose) in the
north-west, and is gradually extending eastwards, is confirmed
by conditions on the narrow-necked Tasman peninsula. This
shows a markedly lower frequency of blacks than the adjacent

MENDELISM AND EVOLUTION IO5

zone of the main island: it seems clear that the extreme narrow-
ness of its neck has hindered the spread of the black gene.

It is interesting to note that Tasmania is cooler and nioister
than Australia, so that the similarity to the case of die black
hamster is very close. Humidity also favours melanism in Coereba.

Another case where a phenocontour map of a mutant type has
been plotted, and examination of the map shows that spread of
the mutant is occurring and is being impeded by geographical
barriers, is that of the simplex condition of the teeth in the field-
vole Microtus arvalis in north-central Germany (Zimmermann,
1935). The aberration here consists in the absence of the last
ridge of enamel on the molar. Here the simplex condition occurs
in over 90 per cent of individuals in Schleswig-Holstein, with
zones of decreasing frequency to west, south, and east. The
mountains of central Germany have proved a complete obstacle,
while certain large rivers have obviously hindered the spread oi
the character. This case is genetically shghtly more complex,
since various gradations in the expression of the character occur,
and not only is the character more frequent in the presumed
centre of origin, but also more extreme in type. It is turther w'ordi
noting that the gradient in s/mpiex-frcquency bears no relation
to another character-gradient or dine (see p. 206) within the
species, namely, the gradual east-west darkening and reddening
of the coat-colour across the north German plain.

In the last two cases, the polymorphism may be regarded as
transient, si n ce the species or subspecies appears to be moving
from a condition in which a given character is rare, maintained
only by mutation-pressure, to one where it is imivcrsal, again
apart from the rare and sporadic occurrence of its allele. Tliis is
theoretically to be distinguished from true polymorphism, in
which a state of balance between the contrasting types is indefin-
itely maintained, but it will not always be possible to distinguish
the one condition from the other (Ford, I940<i). Thus Southern
(1939) has shown that the bridled variety of the guillemot {Uria
aalge) increases northwards from 0.5 per cent or lower to well
over 50 per cent of tlie total population, in a fairly regular
gradient. There is, however, no way as yet of telling whether

I06 evolution: THE MODERN SYNTHESIS

the two types are indefinitely balanced against a climatic gradient,
as with the atctic fox, or whether the bridled type is spreading at
the expense of the ‘hiorniar\ The ow^l Megascop,^ shows a centri-
fugal dine between red and normal (grey) forms in U.S.A.
(Hasbrouck, 1893); while that betw^cen dark and light fulmar
petrels is rather more complex (p. 217).

Mather (1941) has recently propounded a possible genetic
explanation of dines in such apparently non-adaptive characters.
He suggests that the gene responsible is linked with one ot two
balanced polygenic combinations within homologous chromo-
somes (see p. 67), aifectiiig some other character, and that this is
related to some environnaental condition. The mean development
of the character will be adjusted by the relative frequency of the
two combinations, and thus the frequency of the linked gene will
also vary in relation to the environmental gradient concerned.
One may add that the same result would be obtained if one of
the genes in the polygenic combination were pleiotropic and also
produced the visible effect (e.g. bridling).

A frequent condition found in nature is that of regional or
geographical polymorphism, when two or more contrasted forms
are confined to different regions. Thus in the moth Spilosoma
mendica (Ford, 1937), the normal condition is for the male to be
dark, the female white. In Ireland, however, the males as well as
the females are white, and this condition (variety rustied) is known
sporadically on the European continent. Such a state of affairs
may represent the final stage of a transitory polymorpliism, in
an area which favours the spread of an alternative type (c£ the
brush opossum, p. 104), or be due to colonization by one only
of the two forms (and see p. 262). Elsewhere, the condition may
represent the end of a dimorph-ratio dine. Thus, in AedpUer
novae-hollandiae (p. 184) such a dine culminates at eitlier end in
an area exclusively inhabited by one of the two forms. Again,
the white form of the palearctic moth
abundant in the cast, decreases westwards and is absent in the
extreme west (Suomalainen, 1941).

We may conclude by referring to die floral dimorphism of
higher plants, even though diat is solely or mainly environmental

ME'NDELISM AND EVOLUTION

107

or developmental in its determination, not genetic. Floral dimor-

phism is concerned with the relation between cross- and self-
polBnation. (See Kemer and Ohver, 1902, for a good general but
somewhat out-of-date account; Uphof, 1938, for cleistogamy;
p. 140 for gynodioedsm; Mather, 1941, for heterostyly.)

Strictly speaking, the term dimorphism should perhaps only
be employed when whole plants of distinct type are found. This
occurs in most cases of gynodioecism, e.g. in the common
plantain Plantago lanceolata, the viper’s bugloss Echium vulgare,
etc., where some plants produce normal hermaphrodite flowers,
while others have only female (pistillate) flowers, which are
sometimes smaller than the hermaphrodite ones; in a few cases
of cleistogamy, such as the balsam Impatiens noli-me-tangere,
where in addition to plants exclusively producing the normal
showy flowers, others may occur bearing only the inconspicuous
and permanently closed cleistogamous flowers adapted solely to
self-poUmation (as well as still others with both types); and in
heterostyly. Mather (1941) has an interesting discussion on the
evolution of heterostyly (see also Mather and de Winton, 1941).
He points out that an illegitimacy reaction appears to be compul-
sorily associated with heterostyly, and is the chief bar to inbreed-
ing. Homozygous thrum plants, which normally do not occur in
nature, are less viable than the heterozygotes or the recessives
^pms). This reversal of the usual relations of dominance to via-
bility must be due to the accumulation of deleterious mutations,
in the region adjacent to the thrum gene, which will only exist
in nature in a heterozygous condition; this is similar to the
accumulation of loss mutations in the Y-chromosome, which has
led to its almost total inertness (p. 138).

If conditions demand greater stability and therefore increased
inbreeding, in some cases selection may reverse the intensity of
the illegitimacy reaction. Since commercial seed-raisers prefer
selfing, this has occurred with cultivated species, e.g. Primula
sinensis. Here the fertflity of illegitimate relative to that of legiti-
mate pollinations has almost doubled since the experiments of
Hildebrand and Darwin in i|64 and 1877. Another possibility
is the selection of mutants giving homostyle plants which are

108 evolution: THE MODERN SYNTHESIS

then capable of self-pollination. This has occurred in nature
(see pp. 322 , 313).

In a few cases of hetcrostyly, such as certain species of Primula,
the two types of flowers also have different-sized corollas. In
some cases the “pin”, in others the “thrum” type is thus distin-
guished. The meaning of these last conditions is obscure. Kerncr
and Oliver (1902) suggest that the smaller size of corolla is
associated with the type where autogamy is more prevalent, and
cross-pollination accordingly less essential; but this Dr. Mather
informs me is not in accord with experimental facts.

It is also quite logical to use the term dimorphism for types in
which two distinct types of flower are produced on die same
individual plant, as occurs in most cleistogamous forms (species
of Viola, Glechoma, Lamium, Oxalis, Helianthemum, Juncus, many
grasses, etc.). Further, it is perhaps even legitimate to extend the
concept to cover the frequent combination, in one and the same
flower at different times, of definite devices to secure cross-
pollination and self-pollination. This would then constitute what
we may perhaps call a dimorphism in time, since in the great
majority of cases the plant produces flowers which are adapted
to ensure cross-pollination, but if this does not occur, it trans-
forms these same organs into what is virtually a new type of
flower adapted for self-pollination.

Furthermore, in all these cases, a selective balance is at work.
However, the balance is a complex one. In plants whose flowers
change from exogamy to autogamy, as well as those which
produce both showy and cleistogamous flowers, it used to be
supppsed that all that was involved was wliat the experimental
embryologists style “double assurance” [doppelte Sicherung) to
secure pollination. Cross-pollination was assumed to be in some
way better, but if, through lack of suitable insects or other
reason, it chanced not to be effected, then the plant fell back on
its second line of defence, in the shape of self-fertilization. This
in itself would constitute a selective balance of an ecological
nature. The advantages accruing from cross-fertilization arc
offset against the disadvantage of its not being always possible;
the disadvantage of having to produce a second type of flower

MENDELISM AND EVOLUTION

IC^

(or to develop new adaptations within the original flower) is
offset against the advantage of assured seed-production.

Ecological factors may further complicate the picture. Thus
certain species produce solely or almost solely cleistogamous
flowers when growing in deep shade, where few msects are
likely to visit them, but go to die reverse extreme when grown
in sunny open localities. In Viola, cleistogamous flowers are much
more abundant in high latitudes; this is due to a photo-periodic
response (Borgstrom, 1939). Along these hnes, the course of
evolution of the cleistogamous condition is readily envisaged.
Cleistogamy in die strict sense implies a special type of flower
which not only does not open, but shows other adaptations;
usually the petals and stamen are reduced in size, and the pollen-
grains are not hberated from the anthers but germinate in situ.
This condition has doubtless followed on one of “pscudo-
cleistogamy”, where, in certain ecological conditions, the normal
flowers simply fail to open and self-pollination occurs.

This, however, is not the whole story. Autogamy, it is now
realized, is not merely a pis alter. It has certain advantages, in
perpetuating unchanged a vigorous and well-balanced genetic
constitution once this has been evolved. This stability, however,
will only be advantageous so long as conditions also remain
unchanged: in addition, an entirely stable type loses die possi-
bility of invading new environments. Thus the provision of
devices for both cross- and self-fertilization constitutes a balance
between the advantages of plasticity (with its disadvantage of
wasteful production of less well-adapted recombinations) and of
stability (with its long-term disadvantage of absence of adjust-
ment to new conditions). Some forms are exclusively of one or
the other type; but in a large number, probably the majority, of
flowering plants, the two have been brought into baknee, with
consequent floral dimorphism of one sort or another.

A third type of selective advantage concerns the degree of
waste of gametes associated with a particular mode of repro-
duction. This wastage is especially marked in monoecious
plants, and its implications are discussed in reference to the
selective balance involved in gynodioecism (p. 107).

no evolution:- the modern synthesis

It is difficult to evaluate the precise shares of diese various
advantages and disadvantages in floral dimorphism. We must be
content to observe that a selective balance is involved, and that
the dimorphisin is thereby maintained.

In other cases the dimorphism is between sexual and vegetative
reproduction. Dr.W. B. Turrill informs me in a letter that in the
lesser celandine. Ranunculus ficaria, the following types of repro-
duction occur: normal amphimixis, apombds, abundant vege-
tative multiplication (of two types; one variety is wholly or
almost wholly vegetative in its reproduction), and plants with
only male, only female, and only hermaphrodite flowers. DiSer-
ent populations show different proportions of these various
types. Here, in the field of reproduction, we may perhaps have
something akin to the balance of ecotypes in many plant species

(p. 177).

We may in fact conclude that polymorphism always involves
a selective balance, whether it is determiiied genetically, or
environmentally, or internally by the processes of normal
ontogeny, as when two or more kinds of persons or organs,
adapted , to different functions, are formed by the same colony
or individual (for social hymenoptcra see p. 482 11).

Finally in view of its peculiar evolutionary interest as inevit-
ably involving a selective balance and as in some cases leading
by way of ecological regional differences, to sharp geographical
differentiation, genetic polymorphism deserves the most intensive
study, especially in cases where the ratios of the types are geo-
graphically graded, since here we may hope not only to measure
the intensity of the selective forces at work, but also to discover
something as to their nature.

(r) Selection in fluctuating populations

Elton (1930) has pointed out that the customary assumption
of a population approximately constant from year to year is very
far from the truth for many, if not most, species. A stable ‘'balance
of nature'' docs not exist. Fluctuation in numbers, rather than
constancy, is the rule. This fluctuation may be broadly progressive

MENDELISM AND EVOLUTION III

towards increase or decrease, it may be irregular, or it may be
cyclic and regular.

Animal species subject to such regular or periodic fluctuations
include lemmings (Lemmus), snowshoe rabbits {Lepus americmus),
mice, voles, jerbils and other Muridae, foxes {Vulpes, Alopex),
lynxes (Lynx), and other fur-bearing carnivores, certain birds,
such as ptarmigan {Lagopus) in Labrador and nutcrackers {Nuci-
fragd) in Siberia, some invertebrates, land and marine, and
probably certain antelopes and odier larger mammals. The
period of the flurtuation from crest to crest of abundance varies
from 3 to 4 years in the smaller rodents, to lo to ii years in the
snowshoe rabbit, and probably a good deal longer in certain
ungulates. The difference in abundance between crest and trough
may be very great. In the snowshoe rabbit the ratio of high to
low population numbers in extreme cases must reach at least
100 : I. An interesting point made by Rowan (1931, p. 62) for
Canadian birds is that migrant species are not affected by these
cycles. In years when the grouse population has been reduced
to a minimum, the migrants are present in normal numbers.
This fact must, in combination with others pccuHar to migrants,
have important evolutionary consequences restricted to migrant
forms. However, it seems not to be of universal occurrence.

Elton has pointed out certain important evolutionary conse-
quences of these facts. In the first place, both the intensity and the
type of selection will vary continuously during the cycle. During
the period of rapid increase, when numbers arc low and conditions
favourable, the intensity of selection will be very low. During
the peak period, intraspecific selection due to pressure of compe-
tition will be high. Since the catastrophic fall in numbers is
normally due to infectious disease, selection during tliis period
will mainly concern disease-resistance. And in the subsequent
period of unfavourable environmental conditions (for all these
cycles seem to have an external determination) selection will be
concerned with resistance to cold and hunger and similar aspects
of the struggle for existence. To use Elton’s metaphor, the species
is put through a series of examinations, with easy times between-
wliiles, and the different examinations test different capacities.

112

EyOtUTION: thh modern synthesis

Apart from special resistances, such varying selection will promote
a general elasticity of response.

Elton also made the suggestion that periodic fluctuations would
allow greater scope for chance in evolution, since if a rare muta-
tion or gene-combination happens to be present in the much-
reduced minimum population, it will be automatically reproduced
in the same proportion during the period of rapid increase when
the struggle for existence is light and the intensity of selection low.

His views have been criticized on mathematical grounds by
Haldane. However, the proof of the pudding is in the eating,
and the studies of Ford and Ford (1930) make it clear, first that
selection-intensity may actually be relaxed during a period of
rapid increase, and secondly that when it is once more tightened
up, the resultant type may differ from that obtaining in the pro-
ceeding period of abundance. They observed a sharply isolated
colony of the small Greasy Fritillary butterfly, Melitaea aurinia^
for tliirteen years, and obtained records and specimens for a total
period of forty-nine years. The population was increasingly
abundant from 1881 to 1897; it then decreased, and became
scarce by 1906 and extremely rare from 191^ to 1920. A rapid
increase to abundance then took place to 19^4> from when until
1930 it showed a progressive slight further increase.

Variability was slight during the first relatively stable period
of abundance. During the period of rapid increase after scarcity,
however, (I quote from Ford, I934> p- 77)» extraordinary
outburst of variation occurred. Hardly two specimens were alike
and marked departures from the normal form of the species, both
in size, shape and colour, were very common. A high proportion
of these were deformed in various ways, the amount oi deformity
being closely correlated with the degree of variation.*’

With the colony entered on its second period of abundance,
the abnormal types and extreme variants practically disappeared,
and the population settled down again to a urdform type. Thi^
however, was not the same as before, but recognizably distinct.

It seems clear that the relaxation of selection during the recovery
period allowed an excess of variability; and that when it again
became rigorous, the new stable type was slightly different.

MENDELISM AND EVOLUTION

1 13

owing to the accidentah incorporation of different genes. R. L.
Berg (1941) has demonstrated a similar effect in micropopn-
lations of Drosophila melanogaster, aberrations increasing with
decreased intensity of selection.

SewaU Wright (1932), in discussing such problems in more
general terms, concludes that there must be available to most
species a number of gene-combinations all of about the same
survival-value; he compares them to peaks, separated by “valleys”
of intermediate combinations which are less favourable. Normally
it is diffScult or impossible for selection to shift the type from an
estabhshed peak to another, although this might be equally
satisfactory if reached: but when the intensity of selection is
reduced (or when low size of population promotes the accidental
survival of genes and gene-combinations: pp. 58, 199), many
“valley” combinations are realized, the species can cross dryshod
to other peaks, and it will be a matter of chance on which Ararat
the type eventually remains perched when the rising tide of
selection again floods out the valleys.

Elton draws a further interesting conclusion from the facts of
periodic fluctuation. He concludes that it will have promoted
the migratory impulse which is so strong in so many types of
animals when m unfavourable conditions. At first sight, the
existence of this impulse seems hard to explain on any selective
hypothesis, since, in the more spectacular mass emigrations, sudb
as those of locusts, lemmings, or certain butterflies, all, or all but
a negligible fraction of the migrants perish, while the population
is renewed from among those which do not manifest the impulse
and stay in their original habitat One would thus suppose that
migratory tendencies would be strongly antagonized by selection.

However, although such migrations are exceedingly striking
and have thus received a disproportionate share of attention
from biologists, they arc, in fact, but extreme and in a sense
abnormal manifestations of a much more widespread phenome-
non, namely, an impulse to react to unfavourable conditions by
changed behaviour, notably by movement away from an
environment which has become unfavourable. This does not
normally result in mass migration on a vast scale, but in an

II4 bvolution:' the modern ■ synthesis

irregular movement that disperses the population over a wide
area. When lemmings arc scarce in the Arctic, snowy owls
(Nyctea nvetea) descend into north temperate latitudes m se^ch
of food (Elton, 1927, p. 123). When the cedar^conc crop fails m
Siberian forests, the Siberian nutcrackers {Nucifraga caryocatactes
macrorhynchus) leave their usual haunts and may reach western
Europe (Formosov, 1933)- And many quite small and incoiispicu-
otis movements of animal population arc going on a t ie
When migration is of this less extreme type, many mdividuals
which would otherwise die will survive temporarily m regions
beyond the normal range of the species and be able to return
later to their original habitat, while others may survive y
reaching and remaining in other parts of the normal range. In
addition, some mdividuals may be able to survive md to rem^n
in areas outside the normal habitat of the species, either by
adopting slightly different habits and so colonizing different
habitats within the original geographical range, or by colonizing
areas outside this range. This extension of habitat may in the first
instance be dependent on a non-mherited modification of
behaviour, mutation and selection later stepping in to fix the
change gencticaUy (the ‘organic selection” of Baldwin and
Lloyd Morgan, pp. 304, 523); or genetic variants may find them*
selves in surroundings to which their constitution is better adapted
than was the normal environment of the species (pre-adaptation,
see p. 449). In either case, migration will have been advantageous
to the species as well as to the individual.

Elton (1930, p, 52) draws an illuminating comparison between
the sexual process and the migratory urge. Both are exticmely
widespread, and both confer additional evolutionary plasticity on
a species. The sexual process enables the species to exploit to the
fullest extent the mutations, old and new, wliich are carried by
the species or wliich crop up during its evolutionary career, by
making possible every kind of recombination of them. The
migratory impulse, in relation to nntavourable conditions, has
a precisely analogous effect, in increasing the range of environ-
mental conditions with which any genetic variation that exists
can be brought into contact. The two are complementary and

MBNDEiISM AND, EVOmilON II5

often mutually reinforcing processes, and both have their most
important function in times of stress.

Fluctuations in numbers, both irregular and periodic, thus may
have important evolutionary consequences.

7. MUTATION AND EVOLUTION

There remains the difficulty that most mutations so far investi-
gated are deleterious. If mutations are the raw material of evolu-
tion, it is clearly not enough that they should be as it were sterilized
and rendered innocuous; some of them must sometimes be, or
become advantageous. However, this also is not so serious as
at first sight appears (pp. 68 seq.). Since the gene-complex is an
elaborately co-ordinated system, any changes in it are much more
likely to act as defects rather than as improvements. Further, the
larger the change the less likely is it to be an improvement; and
inevitably the geneticist will detect large changes more readily
than small. The detailed analysis of the last ten or fifteen years,
however, has revealed large numbers of gene-differences with
extremely small effects, down almost to the limit of detectability.
It is not only possible but highly probable that among these are
to be sought the chief building-blocks of evolutionary change,
and that it is by means of small mutations, notably in the form of
series of multiple allelic steps, each adjusted for viabiHty and
efficiency by recombinations and further small mutations, that
progressive and adaptive evolution has occurred. Indeed, in cases
where fertile species-crosses are possible, this contention has been
definitely proved, as for instance by the prevalence of multiple-
factor (polygenic) differences, each factor with only a small
effect, as the basis for specific difference in Antirrhinum (Baur,
1932), Phaseohs (Lamprecht, 1941), cotton (Gossypium; Silow,
1941), etc., and in wolf-dog crosses (Iljin, 1941). Many sub-
specific characters have a similar genetic basis, e.g. in the plant
Camelina sativa (Tedin, 1925), in deermice {Peromyscus), gipsy-
moths {Lymantria), etc. (see Dobzhansky, 1937, Chapter 3).
Specific differences in Drosophila depend on many single genes,
often grouped in polygenic systems (pp. 35* ^eq.; Matiicr, 1941)-

Il6 EVOLUTION; THE MODERN SYNTHESIS

The very krge number of genes with small effects involved in
the inheritance of quantitative characters has been stressed by
“Student”. With reference to Winter’s experiments (1929) on
oil-content in maize, R. A. Fisher (igiib) writes: “all commercial
varieties must be segregating in hundreds, and quite possibly
in thousands^ of factors.” With this amount of available variance,
Winter was able to select high and low lines differing sixfold in
oil-content. Sdow (1941) estimates that in cotton {Gossypiutn)
the closest species difer in over half their genes.

It must be admitted that the direct and complete proof of the
utilization of mutations in evolution under natural conditions
has not yet been given. Even the case of industrial melanism,
apart from its concerning the results of man’s interference, will
not be complete until the induction of melanic mutations has
been finally disproved. On the other hand, a complete and direct
demonstration is inevitably very difficult to provide. The muta-
tions concerned will normally have small effects. Thus the species
concerned must be easily bred, and should have been subjected
to detailed genetic analysis: otherwise small mutations will not
be detected. The species must then be foflowed through a period
of evolutionary change, and during this period selection must be
proved to have been operative on certain mutations.

Thus it is inevitable that for the present we must rely mainly
on the convergence of a number of separate lines of evidence
each partial and indirect or incomplete, but severally cumulative
and demonstrative.

These different partial lines of evidence may be s ummarize d
as follows: —

(i) The existence of small mutations. Tliis has been proved in
every organism subjected to detailed genetic analysis. While
most of these are deleterious, it should be remembered that
reverse mutations to wild type have frequently been demon-
strated, both “spontaneously” and under the influence of the
same agency (e.g. X-rays) used to induce the original mutation.
Thus it caimot be maintained that the process of mutation is of
its nature deleterious, since -he “abnormal” can mutate to the

MENDELISM AND EVOLUTION II7

“normal” Position-effects due to small sectional rearrangements
(p. 92) must for our purpose be included in this category.

(2) The existence of mendelizing variations of small extent consti-
tuting the differential characters of subspecies and species. This has
been shown in many cases, though it can, of course, only be
demonstrated for species where fertility and segregation occur
after an inter-specific cross. However, the cases of this are fairly
numerous (see Haldane, 1932a, Chap. 3; Goldschmidt, 1928,
Chap. 15; Dobzhansky, 1937, Chap. 3).

This point is important, since the presumption is very strong
that all mendeHzuig variations owe their origin to mutation.

(3) The existence of selectiotu-pressure (gainst small unfavourable
mutations. All cases of reduced viability in culture and of elimin-
ation of deleterious mutants in nature fall under this head. One
of the best proofs is the low incidence of mutant genes in the
sex-chromosome of wild-caught individuals as compared with
their incidence in the other chromosomes (autosomes), as dis-
covered by C. Gordon (1936) and by Dubinin and his co-workers
(1934, 1936) in Drosophila.

Recessive mutations in the autosomes will not exert their
effects unless in double dose, and they cannot occur in double
dose unless two individuals heterozygous for the gene happen to
mate, which will be a very rare event. Sex-linked mutations, on
the other hand, will immediately exert their effects on a number
of males, since these possess only a single sex-chromosome, so
that any recessive genes located in this chromosome can exert
their effects in single dose, not masked by their normal allelo-
morphs. If the effects comprise reduced viability, selection wdH
at once be brought into play and will tend to eliminate the gene
from the constitution of the species.

Thus both the recessivity of most mutations (pp. 75 seq.) and
the scarcity of sex-Hnked recessives are consequences of selection.

A special case is the proof by Gerould (1921) that the normal
grass-green larvae of the butterfly Colias philodice enjoy a selective
advantage over the blue-^een recessive mutant type in relation
to the attacks of bird enemies, no doubt- on account of their
dose resemblance in colour to the food-plant.

ii8 evolution: the modern synthesis

(4} The existence of mutations which from the outset are non-
deleterious, and especially of those which are potentially favourable. In
view of the recvirrent nature of mutation, it is extremely unlikely
that in an experiment mutations should arise which are markedly
favourable at the outset, in normal conditions; for in most cases
such mutations would long previously have been incorporated
in the constitution of the species.

Of mutations which appear to be potentially favourable—
i.e. capable of being immediately utilized by selection in certain
conditions {see pp. 52, 449 ff.)— there are numerous examples.
"We may mention the mutations in seed-weight of beans found
by Johannsen (p. 52); those modifying hooded pattern in rats
(p. <55); the mutation altering temperature-resistance in water-
fleas by Banta (p. 52); and tliat in tobacco adjusting flowering
to a different rhythm of light and darkness (p. 52). Some of the
mutant genes found by Dubinin et al. (1936) in wild Drosophila
might readily increase (or even become the “normal type”)
under sUghdy altered conditions. The higher variability of
abundant species demonstrates this process in action (p. 58 n.).

Zimmermann (1941) has foimd numerous recessive genes in
the heterozygous state among populations of wild rodents, not
only house-mice {Mus m. musculus), but also field mice (Apodemus)
and voles {Cleithrionomys). Though some of these were for gross
abnormalities, and others for partial albinism or spotting, stfll
others determined characters which might readily be utilized
in normal evolution, e.g. a darker type of agouti. In one case
{Cleithrionomys g.glareolus) a. dominant gene was found, changing
the normal red of the back to the brown character typical of the
alpine subspecies C. «a^cr/.

An interesting example from domestic animals is that of
fiizzled fowls. These have a pecuHar plumage, with upcurled
feathers which do not retain heat well, and are at a great dis-
advantage in temperate climates. The condition depends on
mendelian genes (F. G. Benedict, Landauer, and Fox, 1932).
In tropical cHmates, however, as in West Africa, the breed is
extremely common: here the frizzled plumage is an advantage,
since it enables the birds to keep cooler than normal birds

MENDELISM AND EVOLUTION

II9

(pp. 63, 76; Haldane, 1935; Landauer and Dunn, 1930; Laiidauer,
1937; Landauer and Upham, 1936).

(5) The existence of genetic polymorphism within a species. Tliis,
as we have seen (p. 97), can only occur where there is a selective
balance. Since both the visible and the lethal characters involved
are known to mendeiize in all cases properiy investigated, the
presumption is that they always do so, and the further presumption
exists that they owe their origin to mutation.

Beautiful examples of the action of selection in causing the
spread of favourable mendelian character are seen in those cases
of genetically-controlled polymorphism where one type, when
freed from its linked lethal, has ousted the other (p. 98), as well
as in those where a mutant or rare aUele spreads in certain environ-
mental conditions, as in the industrial melanism of moths, and in
other cases (pp. 94, 104, 203).

Polymorphism must be distinguished from normal variabihty,
however large, grouped in a single normal curve of error, or at
least one without sharply defined modes. When, however, wide
normal variabihty exists, it appears, in so far as it is genotypic,
to depend on mendelizing factors and their recombinations, since
when a cross is made between extreme variants, the F2 is much
more variable than the Fi. The adaptive reasom for the existence
of high variabihty of this unimodal sort are unknown, though it
would appear that in some cases they must exist. Possibly it
supplies the same kind of plasticity in relation to a wide range of
environmental conditions as is found in plant species with numer-
ous intergrading ecotypes (p. 275).

( 6 ) The effect of variation of conditions in altering the incidence oj
selection onfd^ mutations, or (b) naturally-existing genetic differences.
In some cases mutations, which in what may be described as
normal conditions are deleterious, may become advantageous in
other conditions.* A good example is that of the vestigial-winged
mutant of Drosophila, studied by Spencer (1932)- In conditions
near the optimum, vestigial is much shorter-hved than wild-
type. But if vestigials and normals are kept together without food

* When this is so they fall conveniently under the heading of pre-adaptations;
this subject is expanded in Chapter 8, p. 449*

120

EVOiOXION: THE MODERN SYNTHESIS

and water, the vcstigials survive longer. Thus in environments
which occasionally become very unfavourable the vestigial type
might even oust "be normal. It is worth noting that the advantage
or disadvantage concerns the viability characters; the size of the
wings would tiius be a correlated character of no immediate
selective value (pp. 63, 206). On the other hand, reduced size of
wings may have a direct selective value in certain conditions.
Thus L’Heritier. Ncefs, and Teissier (1937) also working with
vestigial, found that flies with this character survived better than
wild-type Dfosophiltt when subjected to constant wind. Tliis, as
they rightly conclude, has a bearing on the prevalence of bisects
with reduced or fimctionless wings on oceanic islands (p. 45 .'>)-

Variation in the environment often leads to selection of certain
types from among the range occurring naturally. This may refer
either to continuous or discontinuous variations. An example of
the former is the case described by Harrison (i920ii) of tlie selec-
tion for depth of pigmentation in the moth Oporitiia aututnnata.
The relative abundance of lighter and darker forms in a dark
pinewood and an acyacent light birchwood was quite different,
and so, but inversely, was the intensity of selection, as revealed
by die number of wings left by birds. The result was that in the
dark environment the darker types had become sixteen times the
commoner, while in the light environment die lighter types were
six times more abundant than the darker. (See also p. 469) ■

As an example of selection between sharply-delimited types we
may mention the experiments of extreme interest carried out by
Sukatschew (1928) on pure lines in dandelions (Taraxacum). He
found that altering the density of total numbers of plants per
plot might completely alter both the survival of the seedlings and
the fertiMty of the survivors, so that a pure line which was inferior
in one set of conditions would oust the rest if the conditions were
dianged.

We may also consider selection as between related species.
Here, similar results to those with varieties of dandelions have
been obtained by Timofeeff-Rcssovsky (1933) with the compe-
tition between die larvae of two species o£ Drosophila at different
temperatures; by Tanslcy (1917) on the varying results of

MENDELISM AND EVOLUTION

I2I

competition between two species of bedstraw {Galium) accordit^
to the type of soil on which they are growing ; and by Beauchamp
and Ullyott (1932) on the decisive effect of temperature in bring-
ing about the selection ot one or other of two species of compet-
ing planarian worms.

Sukatschew’s detailed analysis is entirely in accord with the
elaborate ecological work of the Stapledon school, showing the
effect of varying intensity of grazing on the survival of different
species and strains of pasture plants.

In bacteria, the alteration of type with culture-medium appears
not to depend upon any lamarckian or modificatory effect, but
upon the selection of variants (to use a non-committal term),
though the method of origin of these is still obscure. Again,
selection has different effects on different pure lines of yeast
according to conditions (Cause, 1934).

The diminution in size and other changes which occurred over
a period of 150 years in a stock of horses placed on Sable Island,
Nova Scotia, appear almost certainly to be due to selection m
relation to the somewhat unfavourable conditions (Gates, 1930).
This doubtless has a bearing on the evolution of dwarf forms of
large mammals on islands or near the limit of their range, for
instance the very small Spitsbergen race of reindeer (Rangifer
tarandus), the pigmy elephants, now extinct, of Malta and other
■Mediterranean islands, the Corsican subspecies of the red deer
{Cerms elaphus), etc.

(7) The interaction of two or more unfavourable mutations to produce
a neutral or beneficial effect. We have spoken of the cases of the
mutations for red and transparent eyes in Ephestia and for purple
and arc in Drosophila (p. 69). Another case, of a rather different
kind, is that of the recessive facet-notched, which produces a notch
at the free end of the wing in Drosophila. Its allelomorph facet,
also a recessive, produces irregular ommatidia in the eye, together
with a slight irregularity of the wing-types. When, however, the
heterozygous combination of the two is synthesized, it is found
to produce no visible effects. The combination of the two reces-
sive alleles restores the appearance and viability of the wild type
(Glass, 1933, who cites other cases of the same phenomenon).

122 EVOtUTION; THE MOBEHN SYNTHESIS

Such a state of affairs might lead to the establishment of genetically-
conditioned trimorphism.

(8) The effect of selection of the gene^complex in altering the
expression of wutaticfnSy and especially in abolishing their unfavourable
action while retaining other efects. W^e have dealt with numerous
cases of this phenomenon earlier in the chapter, both as regards
natural and artificial selection. The most striking cases arc
eyeless hi Drosophila (p. 69), the crest character in fowls (p. 73 ) stnd
the restored viability of red-eyed meal-moths when the mutant
gene responsible is combined with another recessive gene (p. 68).

(9) The existence of genetically-detertnined adaptations. Once
more the presumption is that these, if genetic, have arisen by
mutation. If they are truly adaptive, the presumption is that they
have arisen by selection (see Chapter 8 for a development of this
argument and for examples).

(10) The correlation between the incidence and type of genetically-
determined variations in different parts of the range of a species with
variation in the conditions and with the incidence and type of selectiotu
Some of the best examples concern polymorphic mimetic
butterflies. We have first the adjustment of the frequencies of the
different mimetic forms to the frequency of their respective
model species; secondly, the adjustment of the pattern of the
separate forms to the geographical variation of the models, or to
the replacement of one species of model by another species ;
and thirdly, the relaxation of close mimetic resemblance in areas
where the mimic outnumbers the model (p. loi). None of these
phenomena can be explained except on selectionist grounds.
Another equally good example concerns the replacement of the
‘^normal” type of the species in certain parts of the animal s
range by a type which remains rare in other regions, c.g, black
Coerebay black hamsters, black opossums, voles with simplex
type of teeth, melanic moths, bridled guillemots, etc. In all such
cases, there are general grounds (p. 97), and in some cases particular
grounds (p. 95) for believing that selection is at work.

These various lines of evidence all converge to support a
neo-mcndclian view, some showing that small mutations occur,
others that selection is active, that some mutations arc potentially

MENDEI.ISM AND EVOLUTION I23

beneficial, that through selection of the gene-complex, mutations
can be adjusted to the needs of the organism, and that adaptations
are genetically determined and vary in type and accuracy with
direction and intensity of selection.

Three further general points of considerable importance must
be mentioned. In the first place, R. A. Fisher (ipsoa) has provided
mathematical proof of the interesting theorem that the combin-
ation of mutations to provide adaptive improvement in an organ
or process in which the harmonious adjustment of many inde-
pendently varying characters is required, is much more readily
effected by mutations with small than by those with large
effects.

Secondly, both Fisher (1932) and Haldane (1932a) have shown
the enormous superiority, in the hght of existing knowledge, of
selection to other suggested agencies of evolutionary change,
such as true orthogenesis (Chap. 9, p. 509).* Even if genes were
to mutate repeatedly in the same direction, this could have no
evolutionary effect unless they had no influence on viability and
general fitness. A reduction of one-tenth of one per cent in
viability would result in adverse selection which would override
mutation at the highest rate ever yet observed in nature. Simi-
larly, if a mutation caused an increase of viability of only o*i
per cent or over, its spread would of necessity be mainly due to
favourable selection. The same argument appHes to the slow
accumulation of lamarckian efiects postulated by some believers
in the inheritance of acquired characters: if this is so extremely
slow as to escape detection in the course of an experiment covering
a few generations, as they often assert, it would be overridden
by selection-pressure whenever any but the most trifling differ-
ences in viability existed (pp. 457 seq.).

Thirdly, variability varies inversely with selection-pressure
(pp. 324 seq.). The butterfly Papilio Jardanus provides a striking
example. This possesses several polymorphic female forms. Most
of these are mimetic and highly invariable, except when, for
special environmental reasons, they are able to live outside their

* Haldane (op. cit.> p. 142) qualifies this view by pointing out that the paths
open to selective guidance are limited by the nature of variation. This restrictive
or subsidiary orthogenesis we shall discuss later (p. 510).

124 evolution; the modern synthesis

modek’ range. The rare form dionysos, however, is non-mimetic,
and highly variable {Ford, 1936).

In the field we are discussing, of the relations between genetics
and evolution, perhaps the most important single concept of
recent years is that of the adjustment of mutations through
changes in the gene-complex. Before this had been developed
by R. A. Fisher and his followers, notably E. B. Ford, the effect
of a mutation was assumed to be constant. A given mutation, we
may say, made an offer to the germplasm of the species, which
had to be accepted or declined as it stood. And the data on
laboratory mutants at the time indicated that the great m^ority
wotuld have to be declined.

To-day we are able to look at the matter in a wholly different
way. To continue our metaphor, the offer made by a mutation
to the species is not necessarily a final offer. It may be merely a
preliminary proposal, subject to negotiation. Biologically, this
negotiation is effected in the first instance by recombination and
secondarily by mutation in the residual gene-complex. It can
lead to a marked alteration in the effects of the mutation, which
may make the proposal acceptable to the organism.

Ten years ago evolutionary change, on the neo-mendelian
view, depended on the co-operation of two processes only — the
presentation of ready-made building-blocks by mutation, and the
utilization of certain of them under the influence of selection.
To-day we have been brought to reaHze that a third process
is at vvork — change, primarily recombinational, in the residual
variability of tbe gene-complex; and this can shape the budding-
blocks so as to enable them to fit in better with their neighbours
and with the general plan of the building.

Thus evolutionary change, in so far as Darwinian, is not due
simply to the co-operation of mutation and selection; a third,
intermediate agency is involved in tlie shape of the residual
variability of the spcdcs. Adjustment intervenes between
presentation and acceptance.

CHAPTER 4

GENETIC SYSTEMS AND EVOLUTION

1. The factors of evolution p. 12$

2. The early evolution of genetic systems . . . . p.

3. The meiotic system and its adjustment . . . . p. 136

4. The consequences of polyploidy p* 143

5. Species-hybridization and sex-determination: con-

clusion p. 146

I. THE FACTORS OF EVOLUTION

A discursive treatment of mutation, as adopted in the previous
chapter, has at the present time a certain historical justification.
Darwin’s theory of the mechanism of evolution was extremely
abstract and generalized. Next to nothing was knowri in Darwin’s
time of the nature of variations or of the mode of their inherit-
ance, let alone of their differences in various groups of organisms.
The idea of selection remained equally generalized. Darwin
admitted but two types of selection, natixral and sexual. We now
realize that there are many kinds of selection, often antagonistic
in their effects, and not all operative in the same way on all
organisms.

Finally, Darwin had little inkling, apart from his reference to
the greater variability of abundant species, of the evolutionary
effects of differences in the nature of the evolving groups. We
now know, however, that not only these, but also differences in .
environmental conditions, may be of the greatest importance.

The biggest blank on the evolutionary map, however, con-
cerned variation and its inheritance. The theory of mutation on
a mendelian basis is the first adequate attempt to fill the gap. It
has met with great resistance, and Iras itself developed almost
out of recognition during its rapid growth from its beginnings
only a few decades ago. There is thus every reason in the present
state of biology to devote a chapter to mutation treated broadly

126 evolution: the modern synthesis

as embodying Darwin’s shadowy ''heritable variations” and as
representing the raw material of evolution in a generalized way.

It is probable, however, that writers of books on evolution
ten or twenty years hence will adopt a different method. They
will begin by describing the nature of the physical basis of inherit-
ance, its modes of change by mutation of various kinds and at
various speeds, its remarkable general uniformity in all cellular
organisms, and its important variations in detail. They will then
point out' how the nature of this mechanism governs or limits
the evolutionary process, and how its variations affect the mode
of evolution of their possessors. It is impossible for higher
animals, whether arthropod or vertebrate, to evolve in the same
way as do higher plants,. owring to differences in their chromo-
somal machinery: non-cellular and non-sexual organisms such
as bacteria have their own evolutionary rules.^

It is not only the cytological mechanism of heredity, however,
which influences mode of evolution: to use Darlington’s useful
phrase, there is involved the whole genetic system, meaning by
this not only the chromosomal machinery, but the type of
reproduction. Parthenogenesis, hermaphroditism, self- or cross-
fertilization, in- and out-breeding — aU introduce their own
modifications.

Recognition of this fact broadens out into recognition that
mode of life in general has its influence on evolutionary differ-
entiation. A wide-ranging type will develop a different genetic
structure (here we borrow a phrase from Timofeeff^Ressovsky,
1940) from one with limited powers of dispersal. Thus wc need
an index of genetic mobility, or of its inverse function, isolation.
The spread of genes will be different in linear populations, as in
those inhabiting rivers or shore-lines where range is essentially
unidimensional, from what it is in the usual two-dimensional
species (Sewall Wright, 1940, p. 172); it will be different, as
Sewall Wright (1931, 1932) has also shown, in small and in large
populations.

Competition by males for mates or for territory will have

* These topics are discussed at greater length in recent books, such as Darling-
ton’s The Evolution of Genetic Systems (1939).

GENETIC SYSTEMS AND EVOLUTION

127

evolutionary results, some of them rather unexpected (see R. A.
Fisher, 1930^, Chap. 6; Huxley, 193 so will competition
between pollen-tubes in the higher plants, or that between Htter-
mates in higher animals (see Haldane, 1932a); so will the intensity
of general competition, whether exercised tlirough predators or
rivals or through the inorganic environment (see pp. 324,426, 469).

Nor win general organization and mode of development be
without its evolutionary consequences. The meristematic growth
of flowering plants permits a fuller evolutionary utilization of
many types of mutation than is possible to higher animals. In
animals, allometric growth has evolutionary consequences which
in their turn must be difierently adjusted according to whether
general growth is limited or. unlimited (Huxley, 1932; Gold-
schmidt, 1940; de Beer, 19403). The simple fact that most genes
must act by affecting the rate of developmental processes is
reflected in the evolution of vestigial organs, in recapitulation,
in neoteny (see Chap. 9, § 6; and de Beer, 1940b).

The nature of an organism thus influences tlie mode of its
evolution. This appHes at every level. Within the individual, the
microscopic machinery of genes and chromosomes, the mode
of cellular aggregation and tissue-growth; at the individual level,
the type of reproduction, the way of Hfe, the level of behaviour,
the method of development; beyond the individual, the size and
structure of the group of which the individual is a unit, and its
relations with other groups — all these, and many facts besides,
have their evolutionary effects.

Evolutionary consequences of this sort were often so obvious
that they forced themselves upon the attention of the earHest
workers in the field. Darwin fiSyi), for instance, was fully alive
to many of the evolutionary implications of differences in sexual
relations in higher animals, and had noted the greater variability
of large species (reference in Fisher and Ford, 1928). In the
present century, more explicit attention has been given to
particular aspects of the question. To take but a few examples,
Muller early (1925) pointed out the restriction on polyploidy
in animals due to their sex-determining meclunism. Wright
(references in Wright, 1940) gave a .detailed mathematical

128 EVOiUTION: THE MODERN SYNTHESIS

analysis of the evolutionary consequences of differences in
population-size. Haldane (1932a) discussed the different selective
effects of different modes of reproduction, such as the develop
ment of a neuter caste in social hymenoptcra or of polytocy in
mammals; Huxley (1932) pointed out some evolutionary conse-
quences of differential growth.

However, there has been hardly any attempt to survey the
problem as a whole. Darlington’s Evolution of Genetic Systems
(1939) is a notable essay in this direction, though limited to chro-
mosomal and reproductive mechanisms (see also Darlington,
1940); and Goldschmidt’s Material Basis of Evolution attempts the
same for modes of development. A small but increasing number
of writen realize that such a general approach is not only possible
but necessary. Comparative Evolution is destined to become as
important a branch of biology as Comparative Anatomy.

hi any such general survey, other aspects of evolution would
demand the same comparative treatment as that accorded to
genetic systems and other peculiarities of the evolving organism.
The generalized treatment of selection, as originally developed
by Darwin and redrafted on a mendeUan basis by R. A. Fisher
(1930a), must be particularized. Darwin (1871) made a significant
leginning in his separation of sexual and natural selection, and
^ialdane (1932a) has carried the process a stage further by dis-
tinguishing various fomis of intraspecific from interspecific
selection. The analysis cotild, however, be extended on a fully
comparative basis, with every effort to introduce quantitative
treatment at the same time. Selection will act differently in auto-
polyploids because of the reduced availability of recessive muta-
tions (p. 143). The balance between selection, mutation, and
chance recombination will be quite different in large and in small
interbreeding groups, the difference in some cases being so great
that mutation may exert a directive effect (Wright, 1940a.
his p. 173). Certain types of reproductive mechanism or popula-
tion structure may lead to an immediate rapid differentiation and
evolutionary success, to be paid for later by loss of plasticity and
widespread extermination of types, as in CrepU (p. 376), others
to accidental non-adaptivc change or to extinction, as in small

GENETIC SYSTEMS AND EVOLUTION 129

isolated groups (p. 58); male liaploidy will purge the germ-
plasm of many recessives; the development of a reproductive
caste will permit selection for altruistic qualities; familial selection
will promote rapid growth and large size (for both points see
Haldane, 1932b); intcr-malc competition when success may
mean more than one mating will produce male characters of
display or combat which may be deleterious in the individual
struggle for life (p. 426; Huxley, 1938/}). The development of
social life, with consequent inter-group struggle within the
species, may produce the most peculiar selective results, as is
especially to be seen in our own species (see R. A. Fisher, 19300,
Chaps. 10 to 12). Isolation from potential enemies or rivals may
permit unusual specialization, as in flightless island birds, or
encourage variability and degree of adaptive radiation, as in the
fish of certain African lakes or the marsupials of Australia
(p. 324). According to environmental conditions and to the
genetic structure of the group, selection may act cither as a
stabilizing force or as an agent of change, and may decrease or
increase internal variability. We need a comparative study of
selection as well as of genetic systems.

Our last examples remind us that the environment, too, has
its evolutionary effects. Ecology has listed and analysed the chief
types of environments, major and minor habitats, and ecological
niches. It has also pointed out one evolutionary result of environ-
ihental difference, in the adaptive correlation between organisms
and the environments they inhabit (p. 430; Hesse, Alice, and
Schmidt, 193 7). But it has not undertaken detailed analysis of the
eftects of environmental difference on evolution. Here and there a
beginning has been made. The study of the results of the glacial
period— the extinction of some species, the disjunction of others,
the subsequent divergence of their separated portions, and their
behaviour on rc-mccting— has already thrown a flood of hght
on the evolutionary results of violent climatic changes, and
revealed to us that we live in a time when evolution is operating
at exceptional speed (p. 243). The study of the marginal zones
of species is showing that they are often characterized by a
peculiar genetic structure of the population (Vavilov, 1927) or

130 evolution: THE MODEEN SYNTHESIS

by special adaptations (Reinig, 1937), and is also throwmg light on
the evolutionary functions of the harmoniously stabilized gene-
complex (Huxley, I938‘^, 1939^3 and b). The so-called geographical
rules, when analysed in detail, reveal that the sharp discontmmties
of species and subspecies are often superposed upon continuous
gradients of change (cUnes, p. 206), dehcately adjusted to the
external gradients of the inorganic environment (p. 208). Differ-
ences in ecological preference may isolate groups as effectively^
geographical barriers or mere spatial distance, often with the
production of cryptic species fp. 299). Perhaps most important
of all, we arc beginning to realize that the effective environment
of an organism may be and usually is altered by genetic change:
as Darlington (1939) pith% it, “a dwarf bean does not
meet the same environment as a scarlet runner.

Pre-mendehan evolutionary theory arranged its facts and ide^
under three main heads: variation, heredity and selection. This
was necessary to clarify the generahzed theorems of evolution

^natural selection and consequent adaptation; it is still necessary

to-day. But to-day we can go further. Evolution can no longer
be a matter of generalized theorems only: it is itself a major field
for comparative study. The comparative study of the reagent—
the varying, evolving organisms: the comparative study of the
medium— the graded, fluctuating enviromnent: and the com-
parative study of their interaction— the processes of selection and
their consequences: it is along some such lines as these that the
evolutionary text-book of the future must be written.

The time is not yet ripe, however, for such a treatment of the
subject. In this volume, all that can be attempted with regard to
sele'~tir>n and environment will be some incidental comparative
discussion in later chapters. With regard to the nature of the
recent, however, the situation is rather different. The spectacular
advances of cytology in the last two decades now permit the use
of deductive methods on a large scale. Our knowledge of chromo-
somal machinery and of mode of reproduction allows us to make
prophecies concerning genetic detail which may' take years to
verify empirically, and to draw accurate conclusions as to the
type of selection which will operate. Thus it seems worth while

GENETIC SYSTEMS AND EVOLUTION 131

to give a brief summary (largely based on Darlington’s book
(1939) and the relevant chapters in those of Dobzhansky, 1937,
and Waddington, 1939) of the evolutionary effects of differences
in genetic-reproductive systems. In a later chapter other effects
of the nature and mode of development of the reacting organism
will be given (p. 525), though a full treatment of this type of
consequential evolution is not yet possible.

2. THE EABLY EVOLUTION OF GENETIC SYSTEMS

There is an astonishing similarity in the genetic systems of the
great majority of organisms. Their hereditary machinery is
organized into discrete chromosomes of definite size, shape and
genic make-up. The chromosomes divide normally by mitosis,
and at one pomt in the life-cycle undergo mciosis which is
accompanied by crossing-over. This applies to all higher plants
and all higher animals and to many quite lowly forms as well.

The genetic system must have had a long evolution behind it
before it reached what we may call the meiotic stage, with its
elaborate mechanism. Two prior main stages may be distin-
guished, the pre-mitotic and the mitotic, and organisms still
survive which are equipped with genetic systems of tlrese earlier
patterns.

Bacteria (and a fortiori viruses if they can be considered to be
true organisms), in spite of occasional reports of a sexual cycle,
appear to be not only wholly asexual but pre-mitotic. Their
hereditary constitution is not difierentiated into specialized parts
with different functions. They have no genes in the sense of
accurately quantized portions of hereditary substance; and
therefore they have no need for the accurate division of the
genetic system which is accomphshed by mitosis. The entire
organism appears to function both as soma and germplasm, and
evolution must be a matter of alteration in the reaction-system
as a whole. That occasional “mutations” occur we know, but
there is no ground for supposing that they are similar in nature
to those of Hgher organisms, nor, since they are usually reversible
according to conditions, that they play the same part in evolu-

132 evolution: the modern synthesis

tion. We most, in faa, expect that the processes of variation,
heredity, and evolution in bacteria are quite different from the
corresponding processes in multicellular organisms. But their
secret has not yet bcai unravelled (p. 302).

One guess may be hazarded: that the specificity of their
constitution is maintained by a purely chemical equilibrium,
without any of the mechanicat control superposed by the mitotic
(and meioric) arrangements of liigher forms. We may also guess,
with Darlington (1939, p. 121), that the so-called “plasmagencs”
which have been detected in a few higher plants, and which also
seem to be controlled in their reproduction only by a chemical
equilibrium, are survivals, though possibly specialized in their
own way, from the pre-mitotic level of evolution.

The mitotic but pre-meiotic stage is represented to-day by a
few Algae and Protozoa. These may be degenerate in having
abandoned sex and meiosis, or they may never have acquired
them. In any case, the rarity of such cases implies that diis stage
must have been somewhat transitory. Apparently once the
detailed differentiation of the germ-plasm into accurately-
divisible chromosomes had been accomplished, it was compara-
tively simple to alter the timing of the various processes involved
in one cell-division, so as to produce meiosis; and this was fraught
with such advantages that it was all but universally adopted.

It is in any case interesting to reflect on certain peculiarities of
this stage, which must indubitably once have been passed through.
The existence of mitosis, of however simple a nature, presupposes
the need for accurate mechanical division of die hereditary
substance; and this in its turn would not be necessary unless the
hereditary substance were diferentiated into specialized parts
each with their appropriate functions. Thus the mitotic organism
has reached a stage of particulate inheritance, based on spatial
differentiation of the germplasm. Yet it would be improper to
speak of the organism possessing genes, in the sense of definitely
quantized units. Such units may have existed, in the sense that
there was a real division between two adjacent regions of a
chromosome performing different functions. But no method
which we can yet envisage would be able to detect this. Genes as

GENETIC SYSTEMS AND EVOLUTION 133

we know them are mechanically delimited by the lines of poten-
tial crossing-over (p. 49), so that the attainment of meiosis is a
prerequisite for their detection. It may be that another type of
subdivision existed in the chromosomes of pre-meiotic organisms.
If so, we can only say that it is likely to have been profoundly
modified by the superposition of a mechanical jointing, for we
can safely deduce that selection would tend to adjust the two
functions, and convert the mechanically-determined genes into
physiological units as well.

Purely mitotic organisms may have enjoyed a more elaborate
genetic constitution, with its parts more accurately ac^usted, than
pre-mitotic ones. But from the evolutionary standpoint their
behaviour wUl be similar. They are compelled to forgo most of
the advantages of their genetic complexity for lack of the sexual-
meiotic process which permits the recombination of the genetic
units.

The attaimnent of the meiotic stage was thus the most impor-
tant single step in the evolution of genetic systems, comparable
in its evolutionary effects to that due to the attainment of a
c um ulative tradition, and thus of a new form of heredity, in our
own species.

Mere numerical reduction of the chromosomes to prevent
doubling of their number in each sexual cycle could perfectly
weE have been secured simply by a failure of chromosomes to
divide at the first meiotic division. This may have been the first
step towards true meiosis, and something of the sort occurs
(though doubtless secondarily) in the meiosis of the heterogametic
sex of some organisms, such as DrosqpMa.

Such a process would also secure tecombination; but a recom-
bination of whole chromosomes only — in other words a recom-
bination of perhaps a few tens of unite instead of one of hundreds
or even of thousands. The evolutionary advantages of a greater
degree of recombination are so great that this condition, if it ever
existed, has been entirely supplanted by true meiosis, which
implies crossing-over as weU as numerical reduction. It is the
merit of Darlington (see Darlii^ton, 1937) have shown that
crossing-over is not merely the occasional accompaniment of

134 evolution; THE MODERN SYNTHESIS

meiosis, but its invariable and necessary condition. It is through
crossing-over that the bivalents are held together after each
member of a pair has divided into two chromatids: if it were
not for the mechanical union thus provided, they could drift
apart, since attraction only operates between pairs oi homologues,
and could not hold four together. Ghiasmata thus have both a
mechanical and a genetic function; they provide at one and the
same time the cross-junctions needed for the cytolc^ical process
of meiosis, and the sectional separations which give rise to
mendelian recombination of genes.

Meiosis at its first origin was without doubt a process inserted
into the Hfe-cycle immediately after fertilization. Not only are
various primitive organisms haploid throughout their existence
except for the brief moment after syngamy, but it can be deduced
on general grounds that any mechanism for reducing the diploid
number of chromosomes would in the first instance be likely to
come into action as soon as and whenever that diploid number
was reached (Darlington, 1939). Its delayed onset in all higher
organisms must have been secondarily evolved.

Thus diploidy, far from being the normal inevitable condition
we are apt to imagine, was originally an embarrassment to be
got rid of as soon as possible. Measures had to be taken to prevent
the doubling of chromosome-number at each fertilization, and
the simplest way was to reduce it again as soon as possible.

But this apparently commendable promptness had its draw-
backs. For diploidy has a manifest advantage over the haploid
state. It endows the stock with a much higher degree of plasticity
by permitting it to carry a store of recessives in its germplasm.
In a haploid, these would be exposed to the full rigours of selection
in each generation, and the majority of them would be weeded
out In a diploid, thanks to the full evolution of dominance which
diploidy must automatically have brought about (pp. 75 seq.), they
can be carried in evolutionary reserve in reasonable quantity
without being phenotypically expressed, and so exposed to
selection, except in a triJBling number of individuals. If conditions
change, some of them may be employed, either unaltered in ex-
pression, or “improved” by combination with other reserve genes.

GENETIC SYSTEMS AND EVOLUTION 135

So it came about that pure haploidy is now confined to certain
Protozoa and simple Algae, and diploidy has been prolonged
elsewhere for a munber of ceU-generations — about 10 in rotifers,
about 50 in our own species. Diploidy has arisen in two ways.
In the Basidiomycete fungi, the two gametic nuclei do not
fuse, but persist side by sido: as Darlington (1939, p. ii) puts
it, the organism is diploid although all its nuclei are haploid, hi
all other forms, the haploid gametic nuclei fuse to form diploid
zygotic nuclei, which persist through the rest of the diploid phase.

The haploid phase is reduced in all Metazoa to the resting-stage
of a single cell-generation. In plants, this extreme is never reached.
In Bryophyta the haploid stage is the main phase of the fife-
history, both in size and duration. In the Ptcridophyta it is still
independent, but it is now both smaller and briefer than the
diploid. In seed-plants it has become much reduced and confined
to one or two cell-generations. But even here a fundamental
distinction from the metazoan condition remains, notably on the
male side. While the nuclei of spermatozoa appear^ to be quite
inactive genetically, so that they merely transport their freight of
genes without being affected by its peculiarities, this is not true
for pollen-grains, in which particular genes may cause marked
differences in capacity for germination, rate of growth of pollen-
tubes, etc. Doubtless the great majority of genes in higher plants
have no or negligible effects upon the haploid phase, since most
of them will have been primarily selected for their effects on the
much more elaborate diploid phase : none the less, haploid
selection will definitely curtail total plasticity.

It appears that lethals may act during animal meiosis and the
early part of the transformation of the reduced cells into sperma-
tozoa; but once formed the sperms are not affected in their
function by particular genes. In this respect the genetic systems
of animals are more advanced than those of plants, since the
reserve of rccessives which they are able to carry must be some-
what larger. By similar reasoning we may deduce that in regard
to evolutionary plasticity seed-plants must be somewhat superior
to ferns and horsetails, and these in turn to mosses and liverworts
— a conclusion which seems borne out by the facts.

136 evolution: the modern synthesis

3. THE MEIOTTC SYSTEM AND ITS ADJUSTMENT

In general, however, the diploid meiotic system is remarkably
uniform throughout its very wide range of occurrence. Once
established, various internal adjustments are effected between the
parts of the system. In the first place, two conflicting advantages
must be balanced. It is in general an advantage to an organism to
have its normal constitution as harmonious as possible, with its
main genes buffered by modifiers to a maximum efficiency and
viability (p. 67) and mutually adjusted to each other’s activity,
and neighbouring genes harmonized through an optimum
position-effect (p. 85). Even with organisms that show poly-
morphism or excessive variabflity, it will be an advantage for the
central core of the constitution to be buffered and adjusted in
this way. But it is also in general an advantage to an organism
to possess a considerable amount of evolutionary plasticity. The
former is a short-term advantage, giving the closest possible
adaptation to existing conditions, the latter a long-term advan-
tage, comii^ into play if conditions change or even enabling
the stock to extend the range of conditions in which it can tlirive.

Stabilization of internal adjustment can be achieved by decreas-
ing recombination, plasticity by increasing it. Low recombina-
tion is best effected by keeping a large number of genes (here
regarded as mutational units) together in mechanical union
— ^in other words by a reduction in the number of chromo-
somes and a low chiasma-frequency. High recombination implies
the reverse — an increase in chromosome-number and in crossing-
over. Extreme reduction of chromosome-number is difficult
owing to the mechanics of meiosis and mitosis — a. skgle centro-
mere cannot efficiently cope with more th a n a certain length of
chromosome (Darlirgton, 1939, p. 77), and in point of fact we
find a neghgible fraction of species with a haploid number below
4. Similarly, chiasmata normally have an essential function in
keeping bivalents together in the later stages of meiosis, so that
there will be a minimum crossover-recombination due to this
mechanical cause. There is also a mechanical upper limit set to
the number of chiasmata within a single chromosome, which

GENETIC SYSTEMS AND EVOLUTION

137

is revealed genetically by the phenomenon of interference.

For this and other reasons we shall not expect the adjustment
of what Darlii^ton (i 939 &> p. 7?) calls the recombination-index
to the conflicting evolutionary needs of the organism to be at all
close. That it is not so is shown by the variations in mode ofHfe
shown by organisms with similar recombination-indices, and
conversely by the variations in recombination-index shown by
closely-related organisms with similar mode of Hfe.

None the less, some adjustment undoubtedly exists. Chiasma-
frequency in general tends to he below its mechanical upper
hmit, thus reducing possible recombination. On the other hand,
those forms in which recombination is markedly reduced or
absent (e.g. translocation-hybrids such as Oenothera; high auto-
polyploids such 13 S Rumex lapathifoliuin; apomicts, hybrid or
otherwise, as in Taraxacum and Crepis) arc for the most part
doomed to eventual extinction as conditions change and they
suffer in competition with more plastic rivals. Thus in the evolu-
tionary long run the forms with reasonable recombination wfll
smvive to constitute a majority, and at any given time those with
recombination markedly reduced or absent will be new and
relatively short-hved types, and will be in the minority.

Adjustment of the two conflicting needs thus tends to be
cflected in two rather different ways. The need for stabihty will
be met by keeping chromosome-number and chiasma-frcquency
below the maximum possible; the need for plasticity by the
differential extinction of the less plastic types.

Plasticity wfll obviously be affected by mutation-rate as well
as by recombination-index. We know that genes exist which
affect the mutation-rate of other genes (see Sturtevant, 1937, for
summary), even though the number yet described is very small.
Though detailed mathematical analysis is desirable, it is clear in
principle that in a slow-growing organism Hke an elephant or a
tree, mutation and recombination vnll give a much lower pro-
duction of novelty and plasticity than in an insea like Drosophila
with a dozen or more generations annually.

We may thus expea with reasonable assurance diat mutation-
rate also will be in some degree adjusted to evolutionary needs.

138 EVOtUTION: THE MOBERN SYNTHESIS

Oh tlie other hand, here too there is bound to be a great deal of
lag, and much of the adjustment will concern Hfc as a w 4 ole,
operating by the eventual extinction of the inherently less plastic,
instead of concerning the separate species and operating by
changing their separate mutation-rates. Comparative studies in
this field will be of the greatest interest. Meanwhile, we can
point to the much greater observed diversification of herbs than
of trees, of insects than offish or mammals, as a probable evolu-
tionary consequence of liigh plasticity due to more rapid succession
of generations. If the slower-breeding forms have attempted to
compensate for their disadvantage by increased mutation-rate
(see p. 54), the compensation has been a very imperfect one.

Interesting results aire observed when single chromosomes or
chromosomal regions arc debarred from recombination with
their homologucs. The most obvious case is that of the differential
segment of the Y-chromosomes in organisms with a specialiJzed
chromosomal sex-determining mechanism. Here, the homologue
(X) behaves as a normal chromosome, since it can cross-over in
the homogametic sex. Thus not only do Y-chromosomes suifer
a loss of plasticity, but degenerative mutations, if recessive, can
accumulate in them, shielded from selection by their dominant
alleles in the partner chromosome or region. This process has
much more intense effects than mere loss of evolutionary plasticity
and leads rapidly to the region becoming converted into genetic-
ally “inert” material (though it may retain important metabolic
functions: Schultz, Caspersson and Aquilonius, 1940). As Muller
pointed out in 1918, the fact that (part or all of) the Y is debarred
from recombination by absence of crossing-over has allowed
“loss” mutations to accumulate in it until it has become genetically
vestigial. In man and Drosophila it still contzim a few active
genes ; in Drosophila XO males are sterile (abnormal vas deferens) ;
“bobbed” and ever-sporting eye-colour (Goweii and Gay, 1933)
are also Y-bome.

As with somatic vestigial organs, the Y-chromosomc is very
variable in size in closely related species and even witlnn the same
species. Further, once it ceases to contain effective genes, a
mechanical accident at mitosis or mciosis may cause its loss

GENETIC SYSTEMS AND EVOLUTION 139

without this bringing any untoward consequences in its train.
Thus the XO condition has frequently evolved from the XY.

Somewhat different conditions are provided when recom-
bination is equally reduced in aU representatives of a chromo-
somal region. This occurs generally, and perhaps universally, in
the portion of the chromosome immediately adjacent to the
centromere. For simple mechanical reasons, breakage and inter-
change is unlikely or impossible within a certain distance from
the centromere. It is interesting to find that here, too, complete
or considerable inertness has been a frequent result. Evidently,
the shielding of recessives from selection, while it is bound to
accelerate the tendency to inertness, is not the only condition for
it to occur; the complete or almost complete debarring of a
region from recombination may be sufficient condition for it to
become inert.

On the other hand, in other circumstances, it may not. This
is clearly so whenever structural hybridity, whether dependent
on inversion or translocation, has become a characteristic feature
of a species. In the regions adjacent to the centromere, there is
always a reduction in crossing-over. But with sectional rearrange-
ments, only the heterozygote is aflected: crossing-over continues
undiminished in both homozygote types. The heterozygous
combination will therefore not become the sole or the dominant
type unless it is endowed with some countervailing advantage.
Such an advantage may very well accrue from heterosis, since,
granted that an inversion or translocation persists at ah, ks genetic
isolation from its homologous region will force it to diverge
and allow selection to difierentiate it furdier as a regional stabilized
system (p. 362). Once heterozygosity is endowed with selective
advantage, it whl become the dominant or sole phase; and further,
inertness wih no longer be encour^ed in one or both of the
partner regions, since the selective value of the condition depends
on tlie activity of both regions in combination.

In such cases, die loss of plasticity due to lack of recombination
whl be adjusted, if at ah, by the extinction of the ty|«. The great
variability in the degree of interchange hybridity in Oenothera
seems to be evidence of the recent development of the condition

140 ETOtUTION: THE MODERN SYNTHESIS

in this genus; while the state of affairs in Rkoeo, where all the
chromosomes form a ring, and only a single species remaias,
“restricted in distribution and almost invariable in form” (Dar-
lir^ton, 1939b, p. 92), indicates that this genus has been paying
the penalty for its loss of plasticity.

Mather (1940) has recently discussed the evolutionary impli-
cations of monoecism and dioedsm. Complete separation of the
sexes promotes outbreeding, but leads to wastage of gametes
except where, as in higher animals, discriminatory matiag occurs.
In higher plants minimum wastage is best secured by monoecism
combined with devices to prevent self-fertilization. Such sub-
sidiary devices are more readily changed if increased inbreeding
is required (cf. Lewis, 1941, on the flexibility of cytoplasmic as
against genic control of male-sterility).

In gynodioecism, purely female individuals occur as well as
monoecious ones. As in other cases of dimorphism (p. 97), this
rests on a flexible selective balance, determining the proportion of
female plants (p. 107). The advantage of outbred offspring is set off
agairiSt the disadvantage of producing only one kind of gamete.

Inversions and translocations can be considered from another
angle — as one of the aberrations to which the diploid meiotic
machinery is subject. Some of these aberrations need not be
considered here, since, like the production of acentric or dicentric
chromosomes, they are lethal, and so cannot play any part in
evolution. Those which interest us are the types which are
capable of survival and therefore of being promoted from
aberration to norm.

The most obvious of such changes is polyploidy, and the most
obvious fact about polyploidy is its rarity in higher animals as
opposed to its abundance in higher plants. It appears probable that
nearly half the species of flowering plants are polyploids.

Permanent anisopolyploidy is inevitably associated with some
form of non-sexual reproduction, since • triploids and the like -
are incapable of perpetuation by sexual means. Thus, since both
vegetative reproduction and apomixis are very much commoner
in plants than animals, the same is true of anisopolyploids.

This holds also for tetraploids and other isopolyploids. It is

GENETIC SYSTEMS AND EVOLUTION 14I

at first sight less clear why this should be so. However, there are
several reasons. In the first place a single autotetraploid individual
has much less chance of establishing a tetrapioid strain in animals
owing to the rarity of sclf-fertihzation in them; autotetrapioids
arc often sterile with the corresponding diploids, and even if
fertile, the offspring are triploid and therefore sexually sterile. In
cross-fertilized species, vegetative reproduction and apoinbds will
also tend in the same direction. When facultative, they may
multiply the original single tetrapioid many times, and so increase
the chances of two meeting when sexual reproduction supervenes.
This, however, wiU not wholly account for the facts. Auto-
tetrapioids are always initially handicapped by reduced fertility,
since the four homologous chromosomes of a set by no means
always separate in pairs at meiosis, and when three separate from
one, chromosomally unbalanced types result which often display
reduced viabihty or fertility. Autotetrapioids are thus most
unlikely to establish themselves except in types with some form
of non-sexual reproduction — ^in other words, except in plants.*
The rare cases found in animals are usually parthenogenctic (e.g.
the moth Sokiiobia and the crustacean Artcmia).

Allotctraploids are not always restricted to forms which can
carry dn by means of non-sexual reproduction. Thus in certain
moth hybrids (e.g. Pygaera), pairing does not occur at mciosis,
but the chromosomes all remain as univalents wliich divide twice
by mitosis, giving diploid gametes. However, this is irrelevant
from the evolutionary point of view since behaviour-barriers to
mating prevent hybridization in nature.

In various plant species-hybrids, too, pairing fails to occur in
all or ahnost iU chromosomes at meiosis, and unreduced gametes
thus result (though one of the two meiotic divisions is suppressed).
This occurs, for instance, in the celebrated radish-cabbage hybrid
Raphanobrassica. In other cases, however, as in Primula “kewensis,

* In higher plants there are of course great variations in the degree to which
non-sexual reproduction is available. Thus, as Darlington points out (1939,
p. 105), since autotetrapioids are very unlikely to cstoblish themselves in nature
except where such methods of reproduction are available, “by discovering their
occurrence among I>lants, we arc therefore indirectly discovering the degree ot
importance of sexual fertility in the life of the species, a matter to which little
attention has been paid in the past.’V ^ ^ ^ ^

142 evolution: the modekn synthesis

the diploid hybrid is sterile because its chromosomes show regular
pairing. The chromosomes of the two parent species, P.jhribtmda
and P. verticillata, have differentiated sufficiently (largely we may
presume, through translocations) for them to be no longer
functionally equivalent, so that a haploid set consisting of mixed
chromosomes from the two species will not be viable.

Fertility can here only be restored by means of somatic doubling
of chromosome-number, resulting in a tetraploid shoot. The
pairing attraction between chromosomes of the same species is
greater than that between those of different species, so that
diploid gametes each widi two complete or functional haploid
sets arc produced, and fertility is restored. But the production of
a tetraploid shoot is a rare phenomenon: in P. kewensis it did not
occur untU after many generations of vegetative propagation.
Nothing of the sort could have occurred in a higher animal.
Finally, there is the existence in ail or almost aU higher animals
of a chromosomal mechanism of sex-determination. This, as
Muller pointed out many years ago, will in most cases fail to
function in a tetraploid individual, giving numerous intersexes
or other sterile forms. It is true that in the white campion Melan-
drium, which has an XY sex-determining complex, this stdl
functions in the tetraploid (Wamke and Blakeslee, 1939); but
this is due to special quantitative relations between the sexual
valency of the X-chromosome and the autosomes, which are
unlikely to occur generally.

But animals are not wholly debarred from enjoying any of the
benefits polyploidy may bring. They do so through the method
of “repeats” or reduplications of small sections of chromosome,
which bring about what may be called a partial polyploidy. As
M. J. D. White says (1937, p. 107), “that part of their gene-
complex which is tetraploid is possibly less subject to the con-
servative effect of natural - selection and is consequently in more
active evolution.” This was not discovered until recently, and the
extent to which it occurs has, up till now, been investigated only
in Drosophila and other Diptera in which the enlarged salivary
gland chromosomes permit direct examination. However, its
widespread existence' in. these forms, coupled with general

GENETIC SYSTEMS AND EVOLUTION I43

theoretical considerations on sectional rearrangements, makes it
possible for such an authority as Muller (1940) to assert that even
in plants it must have played a more important evolutionary role
than stra^htforward polyploidy.

4. THE CONSEQUENCES OF POLYPLOIDY
We must now consider the evolutionary effects, immediate and
secondary, of polyploidy (pp. 334 seq.). In autopolyploidy, an
obvious immediate effect will be the restricted evolutionary func-
tion of recessive niutations. In most cases these will not exert any
phenotypic effect unless they are present in all four of the homo-
logous chromosomes (though cases exist where three recessives
dominate over one dominant), so that the chance of a recessive
character reappearing after a cross is reduced to that for a double
recessive combination in diploid organisms. This effect increases
by powers of two for successive steps in chromosome-doubling,
so that in high autopolyploids with i6h or higher number of
chromosomes, recessives virtually cease to have any effect on the
organism, either in regard to their single effect or in recombina-
tion. Such forms have their stability-plasticity balance tilted over
in favour of stability, and cannot be expected to survive if
environmental conditions change to any considerable extent.

Meanwhile, however, if repeated chromosome-doublings do
not take place too rapidly, and if the species does not rely solely
or mainly upon vegetative reproduction, counteracting tendencies
are likely to operate which will convert the phylogenetic auto-
polyploid into a functional allopolyploid, at the same time
restoring its sexual fertility (p. 335).

One method by which this is acliieved is by reducing the
number of chiasmata to one per chromosome, which auto-
matically operates to prevent the formation of quadrivalents as
in Tulipa (Upcott, 1939). If any genes exist or later appear which
alter the di&rential pairing attraction of either of the two
members of any chromosome-pair of the original diploid set,
not only will fertihty be restored but some degree of genetic
isolation will arise between the two chromosome-pairs of the
tetraploid and wiU tend to be self-reinforcing. Presumably this

144 evolution: THE MODERN SYNTHESIS

efiect, if strong enough, could operate ah initio, without prior
reduction of the chiasma-firequency, but cases of this are not yet
known. In any case, once such genetic isolation has been estab-
lished, it will open the door to a functional differentiation between
the rwohomologouspairsas regards their externally adaptive effects.

Another method involving adjustment of the number and
behaviour of chiasmata is found in Dahlia, in this case permitting
multiple pairing but compelling regular segregation (see
Darlington, 1939, p. 39).

In any event, sexual reproduction in an autopolyploid implies
natural selection for ferity, and this automatically tends to
convert the species into a functional allopolyploid.

Polyploidy may be expected to increase delicacy of genetic
adjustment in certain respects, by increasing the number of
multiple factor systems. In an octoploid, for instance, every kind
of gene has four homologous representatives. Where all con-
tribute something to a phenotypic result, a very flexible system
is available (see Winge, 1938, on genic replication in general).

In general, the evolutionary consequences of polyploidy may
be roughly compared with those of metameric segmentation in
Annulata: a number of homologous parts are available, between
which functional division of labour is then possible. The fact
that the division of labour is genotypic instead of phenotypic is
irrelevant. The chief difference is that in metamerism the parts
are initially repeated a large and indefinite number of times, and
their later divergence is accompanied by a reduction and definition
of their number. Ih polyploidy, on the other hand, the parts are
never repeated indefinitely, nor, indeed, many times over, and
functional divergence may and does begin when they are merely
doubled. Nevertheless, the analogy is a real one.

In allotetraploids, some degree of functional autopolyploidy
often remains. The reason for the fertility of a form hke, the
tetraploid Primula kewensis is not any inability of the homologous
chromosomes of the two parent species to pair, for they do pair
regularly in the diploid hybrid. It resides in a differential pairing
affinity as between identical and merely homologous chromo-
somes: where tetraploidy has provided two identical cliromo-
somes of each sort, these will normally begin to pair with each

GENETIC SYSTEMS AND EVOLUTION 145

Other rather than with either of their two honiologues from the
other species, and the rapidity of pairing usually does not allow
other chromosomes to be drawn in to form multivalents.

Occasionally, however, some mechanical accident permits the
pairing of non-identical chromosomes or the formation of quadri.-
valents. In such cases, a new type of variation occurs. Segregation
takes place between chromosome-segments of the two ancestral
species (pp. 343, 345; Darlington, 1939, p. 38). Since the ancestral
species may be phylogenetically quite remote, the variational
consequences may be unusual and considerable. Here again, it
is to be expected that selection will automatically step in to
reduce the frequency of such behaviour, since the extreme
variants are likely to be less viable than normal. Accordingly, the
“interspecific segregation” is more frequent in relatively recent
alletraploids, such as Triticum vulgare or Nicotiam tabacum.

The interaction of the two gene-complexes will also produce
various new effects, sometimes unfavourable, sometimes favour-
able (pp. 66, 341 seq.).

It remains to mention one other selective adjustment which
occurs in both auto- and allotetraploids, namely, the abolishing
of much of the physiological effect of polyploidy. Polyploids at
their first formation appear invariably to show some degree of
gigantism, and often vary from the diploid in respect of their
general, vigour, temperature-resistance, and flowering period.
These latter properties have often proved to be pre-adaptive, in
that through them polyploids have extended their range beyond
that of their ancestral diploids. With respect to their gigantism,
however, the general rule has been for this to be reduced or wholly
abolished in the course of evolution. Even octoploid forms exist
wliich are identical in size and appearance wdth the diploid
(e.g. in Silene ciliata: see Darlington, I939> p* 39)- Tbis fact can
only be due to adjustment through selection, and is strong
evidence for rn nrlnding that the mean size of a plant species is
an adaptive characteristic. Darlington suggests that the absence
of polyploidy in certain genera (e.g. Ribes) may be due to the
failure of these secondary genetic adaptations.*

* The absence of polyploidy in. Gyninospenns snd in various Angiospemi
genera is apparently due to its being mechanically impossible where chromcn
somes are too large relatively to cell-size (see Darlington, 1937* P*. 84 )*

146

evolution: the modern synthesis

5. specbes-hybridization and sex-determination:
conclusion

It remains to consider other modifications of the basic meiotic
system, and their consequences. Fertile spedes-hybrids, quite
apart from any questions of polyploidy, appear to be much
commoner in nature among plants than among animals. This is
due to two quite different causes. In the first place, they are much
more readily formed in plants, owing to their passive methods of
cross-fertilization by wind or insects, and to the consequent
absence in them of reproductive barriers based on mating-
preferences, which are all but universal in higher animals.

In the second place, they are more likely to be fertile owing to
the absence of the delicately adjusted sex-chromosome mechan-
ism. In animals, the heterogametic sex in spedes-hybrids is
often wholly or partially sterile {Haldane's rule) owing to im-
balance between the single sex-chromosome derived from one
parent spedes and the autosomes derived from both.*

Where the ranges of divergir^ plant spedes overlap, selection
will normally step in to erect genetic barriers between them. But
where they have diferentiated in isolation from each other, then
fertility on crossing will often remain. Species-hybrids are thus
only likely to occur on a large scale where circumstances cause
spedes to be brought together secondarily. The recent geological
past is a period when this has been happening on a very large
scale, owing to the high degree of range-change consequent on
the alterations of climate since the beginning of the last glacial
period. During the recent historical past an additional agency
promoting spedes-hybridization has been at work, m the shape
of human interference. This may be direct and intentional as
when new spedes are deliberately introduced; or direct and
unintentional, as when they are acddentally transported to new
areas (cf. the production of the hybrid spedes Spartim townsendii

* This is by no means univenal, as is shown by the high fertility of spedes-
hybrids in, e.g., ducks and pheasants. Here, however, the formation of species-
hybrids in nature is prevented by mating barriers. In other cases, such as fresh-
water fish (Hubbs and Hubbs, 1933), spedes-hybridization occurs not infre-
quently in nature, in spite of resultant infertility and upset of sex-ratio.

GENETIC SYSTEMS AND EVOLUTION I47

owing to the accidental importation of an American species of the
genus to Europe; p. 341) ; or indirect, as when species meet owing
to changed ecological conditions caused by man’s interference.
Deforestation in the Balkans, for instance, has provided oppor-
tunities for many plant spepies to meet and hybridize (p. 258);
the extension of cultivation has allowed many weeds of culti-
vation to spread far from their original home.

These two causes taken together have resulted in a degree of
species-hybridization which must be unprecedented in evolution-
ary history. Confining our attention for the moment to fertile
species-crosses, one result has been the production of “hybrid
swarms”. These have been described on a large scale in the New
Zealand flora (p. 355; Allan, 1940), but it is probable that this is
primarily due to the accident of the existence of New Zealand
botanists interested in the problem, and that equal attention
would reveal comparable phenomena in other parts of the world.

Sometimes the hybrid swarm has a mean which is intermediate
between the parents, though of course with excessive variabiHtjr.
In other cases, as with Centaurea hybrids in Britain (p. 258), the
result in some locaHties is the virtual disappearance of one parental
type, save for the modification and enrichment of the other by
a certain number of its genes. In any case, we have here another
example of a mode of evolution to all intents confined to higher
plants.

When species-hybridization is combined with polyploidy and
apomixis, more complex phenomena result. When hybridization
is solely or mainly initial, the result is the formation of numerous
collections of apomicts each centring round a certain mean, as
discussed for Crepis on p. 375; and see Turrill (1938c) for Taraxa-
o</H. Where, however, some of the products of initial hybridization
continue to- cross, we obtain elaborate complexes such as those
o£Rosa, kuhus, etc. (p. 351), in which a number of initial forms
are combined in an evolutionary reticulum. Reticulate evolution
in this form appears to exist only in plants. In animals, it occurs
on a much more modest scale and at a lower taxonomic level,
beii^ usually restricted to the formation of “hybrid swarms”
between a Hmited number (usually only two) subspecies. The

148 E¥OL'0TION: -.the moberm ' synthesis . ,

only case in wMch it has- reached larger scope is in oer owH'
species, where excessive migration, coupled with a breakdown of
purely instinctive mating-barriers, has caused it to operate on a
world-wide scale, producing a phenomenon not found elsewhere
either in plants or animals.

We have several times found the presence of the chromosomal
sex-determining mechanism operating to prevent the occurrence
in animals of this or that phenomenon found in plants. Its presence,
however, also has certain positive consequences. Some of these,
like dosage-compensation of the effects of sex-linked genes, or
indeed the phenomenon of scx-linkage itself, do not seem to
have further evolutionary effects. There are, however, other
effects. For instance, the need for suppressing crossing-over
between the differential segments of X and Y has brought with
it, apparently as secondary consequence, a lower cross-over value
in all chromosomes of the heterogametic sex. The reduction
may be slight, or it may be total as in Drosophila. Unless this is
compensated for by an increase of crossing-over in the homo-
gametic sex, the evolutionary plasticity of the species will be
correspondingly lowered.

The genetic isolation between X and Y leads to a progressive
increase of inertness in the Y, and often to its total disappearance.
Especially in early stages of differentiation, an XY may switch
over to a WZ (female heterogamety) mechanism, as is seen in
cyprinodont fishes. Even in highly specialized forms such as
Drosophila, the role of sex-chromosome may be taken over by
different parts of die whole chromosome-complex in different
species of the same genus (see Muller, 1940; Waddington, 1939).

In such an essentially unstable system situations often arise by
which there are more dito one pair of either X’s or Y’s produced,
and in some cases astonishing complications such as that found in
the fly Sciara (with its elimination of whole chromosomes, in
different lines producing broods of different sexual types, etc.;
see e.g. Metz, 1938). But none of these effects is important from
die evolutionary point of view.

There is another method of sex-determination, however, which
docs have interesting evolutionary consequences, and that is the

GENETIC SYSTEMS AND EVOLUTION 149

method of male, haploidy, where the haploid condition deter-
mines maleness, the diploid condition femaleness. This is best
known in the Hymcnoptera, where it is certainly widespread and
possibly universjd, but has been independently evolved in Thysan-
optera, in two families of Hemiptera, and one of Colcoptera, in
certain mites, and in rotifers (tabulated in M. J. D. White, 1937).
The origin of this mechanism is still obscure, though work on
the parasitic wasp Habrobracon shows that it here operates in
conjunction with female heterogamety and differential fertil-
ization.

Its consequences, however, are obvious enough; all reccssives
will be subject to much more stringent selection through being
robbed of any shelter from their dominance whenever they pass
into the male sex. We might at first sight expect this to result in
the purging of virtually all recessives from all the chromosomes,
in the same way that unisexual haploidy of the sex-chromosome
has led to the virtual absence of sex-hnked reccssives in natural
populations of animals with an XY mechanism (p. 117); which
in its turn would reduce the evolutionary plasticity of the type
to a very low level.

This may have been the effea in certain cases, but it is difficult
to believe that it has occurred in the Hymenoptera, where forms
showing this method of sex-determmation have achieved a great
amount of adaptive radiation and have given rise to some of the
highest and most successful types known among animals. Wc
should hesitate to beheve in the general value of diploidy if it
had been in truth almost entirely sacrificed in this group.

Doubdess male haploidy does very speedily purge the germ-
plasm of obviously deleterious recessives; and this, combined in
social Hymenoptera with the intense mating competition among
the males, must result in a genetic constitution that is extremely
efficient for immediate purposes. Meanwhile recessives can stiU
be carried by the diploid females, which, be it remembered,
usually eiyoy an actual or an effective predominance over the
males, either through the existence of temporary parthenogenesis
or through the social organization in social forms. Wc rhust
accordingly presume that recessives of evolutionary value arc

150 evolution: the modern synthesis

retained in the constitution through’ some form of dosage-
compensation analogous to that which obtains within the X-
chromosome in forms with an XY mechanism.

*****

There remain certain essentially minor types of evolutionary
modification of die genetic system . One of them, that culminating
in the production of true-breedii^ translocation hybrids, Iws
received a great deal of attention owing largely to the historical
accident that its esistence in Oenothera led de Vries to enunciate
his theory, which later proved to be erroneous, of evolution by
large mutations. We now know that this method, for all the
complications of its working and the intense interest which its
analysis ias provided, is both rare and of restricted evolutionary
importance, since it condemns the types which practise it to loss
of plasticity and so to eventual extinction (p. 139; Darlington,

1939).

The analysis could be pushed much further. Facultative and
obligatory apombds, facultative and obHgatory self-fertilization
each impose their own evolutionary consequences; so do the
various degrees of gametic and zygotic mobility and other
fatTors aflEecting the freedom of movement of genes within a
population; so, as we have already pointed out, do the different
intensities of selection to which a type is subjected. Space, how-
ever, will not permit us to pursue the subjert. Enough has been
said to show that each major group, and various minor groups
within the major, will have their own peculiarities of genetic
system and accordingly their own characteristic modes of evolu-
tion. We must not expect plants to evolve along the same lines
as animals. Flowering plants will differ from mosses in their
modes of speciation, trees firom herbs, Hymenoptera from
Crustacea, corals from higher vertebrates. The variety of genetic
systems and of modes of evolution is as important a faa of
biology as the variety of morphological types.

CHAPTER 5

THE SPECIES PROBLEM; GEOGRAPHICAL
SPECIATION

1. The biological reality of species

2. The different modes of speciatiou; successional

species p. 170

3 . Geographical replacement : the nature of subspecies . p. 1 74

4. Clines (character-gradients) p. 206

5. Spatial and ecological factors in geographical diver-

gence . . , . p. 227

6 . Range-changes subsequent to geographical differ-

entiation p. 243

7. The principles of geographical differentiation . . p. 259

I. THE BIOLOGICAL REALITY OF SPECIES

Our third chapter was in the main concerned with the modus
operandi of natural selection in a mciidelian world. We must,
however, beware of thinking that the conclusions thus arrived
at cover the whole field of evolution. There is a danger that the
undoubted and in some ways spectacular success of mathematical
and deductive methods in clarifying our vision and defining the
course of one type of evolutionary process may distract attention
from others of equal or at least of m^or importance.

Deduction and mathematical generalization can only achieve
valuable results with the aid of a firm foundation of fact: the
history of science abounds with examples. Indeed, the history of
this particular subject is especially instructive on the point. The
biometrical school, inspired by Galton and carried on by Karl
Pearson and liis disciples, such as Weldon, applied mathematical
methods of extreme delicacy and ingenuity to the study of
evolutionary problems. But the foundation on which they built
was one of assumptions. When these were not simply erroneous,
like the assumption of blending or of non-particulate inheritance.

152 EVOtUTIGN:"' THE MODERN ^ SYNTHESIS

they were extremely iiicoiiiplete or partial, like the asstuBptioii
of genetic regression, or that of the truth of Galtons so-caUed
Law of Ancestral Inheritance, which have validity only as statis-
tical formulations and even at that are no more than first approxi-
mations. As a result, it is not unfair to say that on the biological
side (as opposed to the mathematical, where definite progress
occurred) no fundamental advances were registered through the
employment of the biometric treatment. This is in strong contrast
with the rapid and steady advances which followed on the dis-
covery of the mendeliaii facts of segregation and reconibinatiom

The more recent fruits of evolutionary mathematics have been
of far greater value, because mathematical treatment has in this
case been applied to a firm basis of fact. This basis of fact, how-
ever, has been for the most part confined to the elementary
behaviour of genes — segregation and recombination*, dominance
and recessiveness and their possible origins; gene-mutation and
its frequency, in relation to total numbers.

There is no doubt that the conclusions deduced from these
data are of extreme importance: but it is equally certain that they
do not cover the whole field. It has been known for some time
that genome-mutations (polyploidy) play a considerable role in
higher plants. Later research has shown that aneuploidy, hybrid-
ization, segmental interchange, and other processes affecting the
chromosomal mechanism of heredity are also of importance in.
plants, and the most recent work on Dtosophila has shown that
many of them have had their part to play in animals too. These
points have been dealt with in the preceding chapter.

So far, almost the only attempt to generalize these facts and
to use them as a basis for large-scale deduction is that of Darling-
ton (1937): it seems clear, however, that this is a necessary next
step. Evolutionary mathematics in the pre-mcndelian era was
litde more than a chimera bombinating in a biological vacuum.
In the transitional period, with which the name of R. A. Fisher
is especially associated, genes have been the grist for its mill. The
time is now Tipproacliing when the chromosomal and genic
apparatus in its entirety, with all the peculiarities of its behaviour,
can be utilized as factual basis.

THE SPECIES PROBtEM : GEOGRAPHICAL SPECIATION 1 53

Meanwhile discovery has already progressed far enough to
show that these peculiarities of chromosomal behaviour are of
great importance in evolution. We may discover eventually that
they have something to say in regard to long-range evolutionary
trends, to the initiation of new evolutionary possibilities, and other
major processes. So far, however, their chief importance appears
to lie in producing diversification through species-formation; and
it is to this process of species-formation that we must now turn.

Darwin himself happened to confuse the issue by calling his
greatest book the Origin of Species, though this is but One aspect
of evolution. Evolution must be dealt with under several rather
distinct heads. Of these one is the origin of species — or, if we
prefer to beg no questions, we had better say the origin of bio-
logically discontinuous groups. Looked at from a rather broader
angle, this problem presents itself as the origin of minor systematic
diversity, including the origin of what taxonomists call varieties
and subspecies, species, genera, and perhaps families. Another is
the origin of adaptations. A third is extinction. And a fourth, and
in many ways the most important, is the origin and maintenance
of long-range evolutionary trends.

It is, of course, true that these all overlap and interlock. A long-
range evolutionary trend cannot take place without involving the
origin and apparently the extinction of many species, or without
involving the origin and improvement of many aikptations.
Most adaptations themselves involve at least subspecific or
specific change, and many subspecific and specific characters are
adaptive. None the less, the distinctions arc real and important.
The origin of minor systematic divenity in general seems to have
little to do with the my or processes of evolutionary change; and,
as various authors have shown (see espectially Robson, 1928;
Robson and Richards, 1936), specific and other minor systematic
characters frequently have no discernible adaptive significance.*
Accordingly, I propose to deal with each of the topics in turn.

* I say discernible. This is partly because much of speciation is concerned with
invisible, physiological characters; partly because taxonomists deliberately prefer
to base their diagnoses on non-adaptive characters; and partly because many
non-adaptive characters are correlated with adaptive ones. But even so, a number
of non-adaptive specific characters would seem to remain.

154 evolution: the modern synthesis

First, tken, we have the problem involved in the origin of
species. As a preHmiaary to that, logic demands that we should
Apfnf the term. It may be that logic is wrong, and that it would
be better to leave it undefined, accepting the fact that all biologists
have a prs^matic idea of its meaning it the back of their heads.
It may even be that the word is undefinable. However, an
attempt at defimition will be of service in throwing light on the
difficidties of the biological as well as of the logical problems
involved.

In the first place, although, as we shall see later, the degrees of
discontinuity represented by good species and by certain types
of subspecies constitute favoured equilibrium-positions in the
process of taxonomic differentiation, so that borderline cases are
rendered less frequent than we should otherwise expect, yet
there cannot be any hard-and-fast distinction between a species
and a subspecies or variety, since in many instances one arises
gradually out of the other in the course of evolution, and it must
often be a matter of taste and convenience where the line is drawn.

Secondly, a very important fact for our discussion, there are a
number of quite difierent kinds of animal and plant species,
difering in their mode of origin and in their biological character-
istics. The remainder of this and of the following chapter wiU be
mainly concerned with amplifying the evidence for this fact and
drawing conclusions from it. Here we will merely mention a few
points. In so far as species are biological units, markea oflf from
related units by partial or complete discontinuities, they may
originate in several different ways (see e.g. Rensch, I939«) : the
most important are the geographical, the ecological and the
genetic. With geographical differentiation, spatial separation is
the primary factor, paving the way for biological divergence and
subsequent discontinuity. With ecological difierentiation the
primary factor is divergence in functional specialization, which
may lead to full speciation with complete biological discontinuity
even within one and the same geographical area. And with
genetical differentiation, the primary factor is some alteration in
the genetic machinery underlying heredity, sex, and reproduction.
This acts at once and automatically, either to prevent inter-

THE SPECIES problem: GEOGRAPHICAL SPECIATION 1 55

crossing between the two types, or to render them or their
hybrid oflfspring partially or whoEy infertile.

Each of these main types of speciation produces species with
somewhat different biological characteristics. Related geographical
species tend to be distinguished by broad and general adaptations
to cHmate, and to lack special genetic or behaviour mechanisms
evolved for the prevention of intercrossing: when geographical
accidents produce complete spatial discontinuity, this will tend
to produce a greater degree of biological discontinuity than
would otherwise have occurred.

In addition, when isolation is relatively complete and when, in
addition, the isolated populations are small, non-adaptive is super-
imposed upon adaptive divergence, often to a marked degree,
chiefly owing to what we have called the SewaE Wright effect,
or drift. Related ecological species tend to be characterized by
detailed adjustment to special habitat and mode of life, and often
by special adaptations to prevent intercrossing. And genetic
species, especially those which are biologically more or less com-
pletely discontinuous from the outset, will owe their success
initially to general and intrinsic characters like vigour, not to
gradually-evolved adaptations, whether general or special;
further, the differences in morphological, and other, “characters”
by which they are distinguished from their closest relatives wiU
often be, relatively speaking, small (see p. 385).

Species will also ^ffer from group to group and from area to
area, both for intrinsic and extrinsic reasons. The nature of the
reproductive and sexual mechanisms found in a group will have
an influence on the nature of species that constitute it. When
asexual reproduction exists, either exclusively or side by side
with sexual, certain possibdities of species-formation are open
which are not available to exclusively sexual forms. Similarly the
typical animal method of sex-determination by dissimilar sex-
chromosomes almost entirely rules out certain methods of
speciation found in plants (p. 142).

Again, sedentary and less mobile forms wdl differ, espcciaEy
as regards the degree of geographical speciation, from more
mobile types (p. 239). And ecological speciation is encouraged

156 EVOIUTION: THE MODERN SYNTHESIS

by a decrease of biological competition (p. 323). Ot course, these
various differences of origin, nature, and environment may over-
lap and combine, so that there will be great variation in the size,
discontinuity, and distinguishing characteristics of species in
different groups and different regions.

It is this fact, of the existence of different kinds of species and
of different degrees of speciation within each kind, which makes
it difficult to give a satisfactory definition of a species, and makes
us sometimes wonder whether the term itself should not be
abandoned in favour of several new terms, each with a more
precise co nn otation. However, we may here reflect that the term
species has a practical as well as a theoretical aspect. It is necessary
for the museum systematist to have some criterion by which he
can allot specimens to the pigeon-holes of named taxonomic
units. Frequendy he has to give an opinion on a few preserved
specimens sent for identification. His work may often have
important practical bearings: it is necessary for practical reasons
to be able to distinguish between a mosquito that transmits
malaria and one that does not, or between two plant species in
only one of which the essential oil is commercially valuable.
Thus, whatever refinements of method he may call to his aid in
regard to favourable material, whatever niceties of ecology,
genetics, or cytology he may wish to evolve in his theoretical
studies, die fact remains that for his practical routine he must
have some rule-of-thumb criterion for distinguishing related
forms and deciding when they deserve separate names. It is
inevitable and right that minor systematics shall be a compromise
between the complexity of biological fact and the logic of
practical convenience.

One of the most important tools of taxonomy is nomenclatorial
terminology. Incomplete or incorrect nomenclature may indeed
involuntarily distort the factual data. For instance, if, as at present,
current taxonomic practice operates almost exclusively by giving
names to areal groups, and does not provide terms for continuous
gradations, then what arc really arbitrary stages in a gradation
win often be given names, implying that they are uniform groups
with a definite distribution (p. 226). The basic theoretical aim of

the species problem; GEOGRAPmCAI, SPECIATION 157

taxonomy is obviously the accurate description of organic
diversification in nature. Although for reasons of convenience it
is desirable to have a general terminology, like that of species and
subspecies, applicable to the majority of organisms, yet it must
be recognized that this does not apply at all in certain exceptional
cases (p. 353), and that it must in many groups be supplemented
by additional terminology. However, although certain new
terms should probably be incorporated into the nomenclature,
reasons dictate that most of such additional termin'^
ology should be purely supplementary, adopted as aii additional
means of analysis for this or that special purpose (p. 405; Turrill,

1938U). ...

A quite reasonable definition of the term species is that given

some years ^o by Dr. Tate Regan when Director of the Natural
History Museum at South Kensington— namely, that “a species
is a community, or a number of related communities, whose
distinctive morphological characters are, in the opinion of a
competent systematist, sufficiently definite to entitle it, or them,
to a specific name” (Regan, 1926). The difficulty with this
definition Hes in the term competent, which is what we have
recently learnt to call the “operative” word. And experience
teaches us that even competent systematists do not always agree
as to the delimitation of species. • j

Furthermore, in view of what we have previously said as to
the existence of different kinds of species, it is clear that the
competence of a systematist in this respect must be in the mmi
confined to groups which he himself has studied in detail, for
other groups may differ in their prevalent mode or degrees of
speciation, or in other characteristics of the species of which they
consist. It is no good asking a systematist who has drawn
experience from a higher animal group such as bnds to apply his
competence directly to a plant group such as the Compositae,
still less to one like the brambles or the roses, in which, as we
shall see (p. 351), wholly different processes are operatmg to

produce group-d^rentiation. . , 1 ^

And even in groups with the same general biolo^cal character-
istics, and therefore the same general type of spedation, expenence

158 evoiution: the modern synthesis

is needed to decide what characters arc of value to the practical
systcmatist in separating his groups. Sometuues these appear
arbitrary enougL For instance, in the group of fossil fish known
as Paleoniscoids, it is customary to use differences of body-scale
ornament as diagnostic of species, those of head-scale ornament
as diagnostic subspecies. In fish, again, the fusion of the lower
pharyngeal bones to form a single plate is used in the perches as
a generic diagnostic, while it is used as an ordinal character for
the order Synentognathi (sec Norman, 1936).

Such examples once more remind us of the pragmatic aspect
of taxonomy involved in the need for quick and simple pigeon-
holing. In general, systematists prefer non-adaptive (or apparendy
non-adaptive) characters as bases for their diagnoses, so long as
they arc readily visible. Such characters are less likely to be
obscured by parallel or convergent evolution in response to
selection-pressure (p. 357). In passing, wc may note that this very
natural preference goes a considerable way towards explaining
the assertions of the non-adaptiveness of speCiation that are made
by many systematists. But what precise characters shall be chosen
as predominantly suitable for classificatory diagnosis must in
each case be discovered anew by experience. What works in one
group may have no pragmatic taxonomic value in anodier, even
though closely related. Chapman (1924) has studied die question
carefully in birds. He considers that hard-and-fast rules should
not be followed. The variability and evolutionary plasticity of the
group and the degree of its adaptabihty in habit, must be taken
into account, and difierenccs in voice and behaviour are to be
regarded as of equal or sometimes greater importance than those
in morphological characters. If so, then even in the absence of
adequate collections throughout the whole range, the systematists
should be able to classify specimens much more successfully by
such comprehensive methods than by rule-of-thumb procedure.

/ None (he less, even when the differences between groups and
the claims of practical pigeon-hoUng have been allowed for, diis
definition of Regan’s must be taken into account, for there is
some reasonable measure of agreement among competent
s3rstematists as to the criteria they adopt for classifyii^ organisms

THE SPECIES problem: GEOGRAPHICAL SPECIATION 1 59

in different species. These arc first, visible (niorphological)
resemblance between members of a group, of such a nature as to
be consonant with the view that the group is actually or potentially
an interbreeding one; secondly, lack of intergrading with other
groups; thirdly, a geographical area of distribution consonant
with the idea of a common ancestry for the group; and fourthly,
where data arc available, infertility on crossing with related
forms.

The first three criteria can be evaluated on the basis of pre-
served specimens and records of their provenance. They may be
modified in various ways according to special circumstances.
For instance, as regards resemblance, experience has taught that
in some cases large differences in appearance arc possible witliin
an interbreeding group. The colour-phases of some birds and
mammals (p. 184) are examples; but the most striking cases arc
those of polymorphic mimicry in butterflies (p. 102). The older
entomologists were shocked at the idea that such diverse types
might belong to a single species. Thus Hewitson (1874) wrote
with regard to Papilio merope (now called P. dardanus) and its
polymorphic female forms, each then regarded as a distinct
species: —

‘‘Mr. Rogers has sent me a second collection of butterflies
from Fernando Po, containing P. merope and P. hippocoon taken
by him in copulation, another illustration of the saying that
'truth is stranger than flction^ I find it very difficult even with
this evidence to beheve that a butterfly, which when a resident
in Madagascar has a female the image of itself, should in West
Africa have one without any resemblance to it at all.’’

But breeding tests have proved that the older entomologists
were wrong.'*^

Systemadsts have also learnt to discoimt occasional mutant
forms, though here again, in the absence of actual breeding

* Actually, the difference between the two sexes of one and the same species
may be far more extraordinary, as in the worm Boncllia, or in certain angler-
fishes. But we are so accustomed to this type of difference that it no longer strikes
us as remarkable, although in point of fact the genetic and developmental
mechanisms by which this difference is maintained shed light on the origin and
maintenance of other kinds of intra-spccific variation such as mimetic poly-
morphism.

l60 EVOIUTION ; THE MODERN SYNTHESIS

experiments, individual opinion must enter into practice (see
Chapman, 1923, 1928; Stresemann, 192^-6-, Bateson and Bateson,
1925). A constant average morphological diftcrcncc from other
groups is thus the first criterion (Regan, 1926), though, as we
shall later sec, it is not an indispensable one, and, as Mayr (1940)
has pointed out, subspecies may diftcr visibly more than do good
species.

As regards intergrading, a number of quite different situations
present themselves.* Wliat we may call simple iiitcrgrading is
shown by subspecies inliabiting a continuous land area, when
tihese intergrade by freely interbreeding in narrow zones at the
margins of their ranges. In some of these eases careful analysis
has shown that there exists a dine or continuous gradient of
change in subspedfic characters, which is gradual within the
main areas of the subspecies, but much steepened across a narrow
intermediate belt (p. 187): it is possible that the majority of eases
of true intergradation will prove to consist in such a steepening
of general gradients of change (p. 209).

Sometimes, owing to physical barriers, there is little or no
interbreeding at the margins of die group-areas. Tliis may lead
to complete discontinuity of type, as with island forms such as
the St. Kilda wren {Troglodytes t. hirtensis), although in other
eases the mean differences between the two populations may be
no greater than when intergrading occurs. In some cases, how-
ever, complete physical and gcnctical isolation may exist with
shght or even no character-difference between the types.

hi still other eases there is an interbreeding zone in wliich,
instead of the phenotypically simple gradation between two not

* The tenn intergrading is here used in the sense of geographical intergrading,
usually along a- marginal zone delimiting populations of distract mean type
(although irregular types of iiitergradation mentioned also occur). Such geo-
graphical intergradation appears always to rest on genetic mixture of types, hi
systematic Htcraturc, however, the term is sometimes used to denote that two
populations of diflferciit mean type overlap in their visible characters, irrespective
of whether one population actually passes into the other by means of a change
in mean character. To avoid confusion, this should rather be styled morphological
overlap. Marked morphological overlap may occur between two quite discon-
tinuous populations (c.g. an island and a mainland form), where accordingly
there is no geographical iiatcrgrading, and genetic intergrading is absent or
negligible.

THE speCbes problem: GEOGBAPHICAL SPECIATION l6l

very dissimilar types, which we have just been considering, we
find obvious mendelian recombinations involving the characters
of two markedly disfinct types on either side of the zone, as in
flickers and other birds (p. 250). If we want a special term, we
may call this a zone of recombination (though we must remember
that recombination must also be at work in the zones of simple
intergradation between subspecies that differ only slightly and
in quantitative ways). When it occurs, it may be taken as evidence
that two groups which have undergone considerable differentia-
tion in complete isolation from each other have later extended
their ranges so as to come into contact, owing to climatic or
geographical changes. A stiH further complication is provided by
forms such as the brambles or the hawkweeds (pp. 352, 372), in
which irregular reticulation, apparently due to widespread
crossing, recombination, and apomixis, occurs between various
main types over a large area and not only along a marginal zone
between the areas of two uniform types.

A quite other form of intergradation is seen when two groups
differ in the percentage of two or more strikingly different forms
or “phases”. Thus the different band- and colour-types of the
snails Cepaea nemoralis and C. hortensis in different propor-
tions in different localities, as do the percentages of white and
blue arctic foxes {Alopex lagopus) or of bridled and non-bridled
guillemots {Uria aalge; Southern, 1939). etc. In some of these
cases, such as the guillemot, there exists a regular geographical
gradation (dine) in the ratio of die two forms (pp. 105, 217),
whereas in others, e.g. Cepaea, the distribution of types appears
to be wholly random. Limiting cases arc also known, wWe a
type exists in two forms in some parts of its area, but in only
one of them in other regions (p. 184.).

Finally, gradual dines in modal character (not in the ratio of
sharply distinct types) may be exhibited over considerable areas
(p. 220). In some cases the presumptive evidence supports the
vipw diat die phenomena are due to hybridization, but is more
often against it. Classical taxonomy has for the most part con-
cerned itself only with the intergradation to be observed in
narrow zones; but, as we shall see later, large-scale dines of

i62 ' ' ^ evolution:, the mobeen: synthisis^.^

various types, Aough of different significance, are probably of

eoual importance. - . i

With regard to the criterion of geographical area, matters^are

in most cases fairly simple. Difficulty arises, however, when there
is considerable crossing between weU-differentiated forms.

Before evaluating these criteria further, we mmt mention me
classical criterion of infertiHty, which of course is not avaffable
for most museum specimens. It was at one time considered that
this was crucial. “Good species” were those which were cither
directly infertile, or yielded infertile hybrids: fertility between
two types proved that they were not species but only vaneties.

This view, however, is no longer tenable. Undoubted speaes
may cross and yield fully fertile hybrids (see Goldschimdt, 1928,
p 392), while forms which are partially or wholly infertile with
each other may be' so similar in appearance as to be barely distin-
guishable (Drosophila simulans and D. meknogaster, p- 333 ; the
two “races” of D. pseuJoobscura, p. 323; certain biological
races”, p. 295; the peculiar “races” of mosquitoes, p. 317; etc.).

Dobzhansky, in his recent book (i 937 > P- 3 10), seeks to over-
come the inherent difficulty of definition by substitutmg a
dynamic for a static concept of taxonomic categories. For him
the species is “that stage of the evolutionary process, at which
the once actually or potentially interbreeding array of forms
becomes segregated into two or more separate arrays which are
physiologically incapable of interbreeding”. The dynamic pomt
of view is an improvement, as is the substitution of incapacity
to exchange genes for the narrower criterion of infertihty: but
even so, this definition cannot hold, for it still employs the lack
of interbreeding as its sole criterion. “Interbreeding without
appreciable loss of fertility” would apply to the great majority
of animals, but not to numerous plants. In plants there are many
cases of very distinct forms hybridizing quite competently even
in the field. To deny many of these forms specific rank just
because they can interbreed is to force nature into a human
definition, instead of adjusting your definition to the facts of
nature. Such forms are often markedly distinct morphologically
and do maintain themselves as discontinuous groups in nature.

CT SPECIES probdbm: geographicae speciation 163

If they are not to be called species, then species in plants must
be deemed to differ from species in animals in every characteristic
save intersterility (see also p. 342).

Dobzhansky is perfectly aware of these difficulties, but is
inclined to minimize them. He concludes that if groups at this
level of evolutionary definition are not to be called species, they
do at least demand some name. This may be granted, yet it may be
preferable to employ subsidiary terminology for such one-criter-
ion categories (cf. the term commiscmm proposed by Danser, 1929),

We have just noted that certain authorities have attempted to
erect infertility on crossing into an absolute criterion of species.
Others have done the same for lack of geographic^ and genetic
intergradation, irrespective of the degree of visible differdice
between the two types. This, indeed, is a common practice of
many American systematists. It is, however, very difficult to
justify any such hard-and-fast rule as a matter of principle, since
it can only be a mere rule of thumb. There are many cases where
the extremes of a chain of intergrading varieties are far more
different than, say, an island and a mainland form which happen
for geographical reasons to be unable to intergrade. It appears
quite illogical to erect the latter to the rank of spedes while
leaving the former as subspedes: the one may be more likely
than th.e other to differentiate later into a full spedes, but that is
another matter. The question has been ably discussed by Chapman
(1924), who emphasizes the need for a broad biological outlook
in minor systematics. Stresemaim (1927) adopts the same bio-
logical standpoint.

As regards geographical variation within the species, modern
practice is tending more and more towards the adoption of the
prmdple embodied in the German term, introduced in 1926 by
Rcnsch, of the Rassenkreis* This may be stated as follows. When
one form is replaced by another very similar form in a different

* For a discussion of this and similar terms see Rensch {i934)- As Reiisch-
points out^ the term Fomettkrvis^ proposed by Kleinschmidt, suffers from various
disadvantages in that he includes under it undoubted species as well as subspecies,
and docs not insist on the principle of replacement. May r (1940) refutes the
view of Kinsey (1937) that the title of species should be given to the lowest
distinguishable systematic category, which will in fact usually be the subspecies.

i 64 evolution: THE MODERN SYNTHESIS

geographical area, the two should be considered as subspecies,
whether they show intergradation or not, unless the difFerence
between, them is so marked that we should be justified in pre-
suming that they would not cross if present in the same area in
nature, or that they or their hybrids would be infertile on crossing.
Even so, we may find our rules inadequate. Sometimes the end
members of a single chain of intergrading subspecies will not
breed together (see p. 244). Such cases merely emphasize the fact
that there can be no sharp line between subspecies and species,
and that discontinuity between groups may arise gradually. The
converse fact that forms showing much less difference in visible
characters than that between undoubted subspecies may hve in
the same area without interbreeding and must therefore be
regarded as good species, shows that we must not make a hard-
and-fast rule on the basis of visible differentiation cither.

In general, it is becoming clear that we must use a combination
of several criteria in defining species. Some of these arc of hmiting
nature. For instance, infertihty between groups of obviously
distinct mean type is a proof that they arc distinct species,
although once more the converse is not true.

Thus in most cases a group can be distinguished as a species on
the basis of the following points jointly; (i) a geographical area
consonant with a single origin; (ii) a certain degree of constant
morphological and presumedly genetic difference from related
groups; (iii) absence of intergradation with related groups.
Where evidence is available, infertility with related groups will
be extra evidence for specific distinctness, but its absence will
not be conclusive as evidence against such distinctness. The
actual absence of interbreeding in nature is in some ways of
greater importance than infertility. The lack of interbreeding
may depend on mere gcograpliical separation, on psychological
barriers, on ecological separation, on difference in breeding dates,
etc. ; but such absence will in point of fact isolate groups, whether
or not in abnormal circumstances they can be made to mate
and their matings arc then fully fertile. The absence of inter-
breeding connotes absence of inter gradation; and botli can be
summed up under the head of isolation. Our third criterion

GEOGRAPHICAL SPECIAHON 165

above, if translated from the terminology of the museum to that
of the field, may thus be formulated as a certain degree of bio-
logical isolation from related groups. When two morphologically
and geographically distinguishable groups will under no circum-
stances produce fertile offspring, the biological discontinuity is
both complete and absolute. When they produce a reduced
number of o&pring, or offspring with reduced fertility, the
discontinuity, though absolute, is partial. When, however, they
do not normally interbreed, though they are capable of free
interbreeding under changed geographical, ecological or other
circumstances, the discontinuity, as found in normal circum-
stances, is complete but relative.

In most cases a species can thus be regarded as a geographically
definable group, whose members actually interbreed or are
potentially capable of interbreeding m nature, which normally
in nature does not interbreed freely or with full fertility with
related groups, and is distinguished from them by constant
morphological differences.

This is in general satisfactory, but difficulties sometimes arise.
These difficulties differ with different methods of species-form-
ation. With geographical speciation, one difficulty concerns the
extent of morphological difference: there are bound to be
borderline cases. Another difficulty arises when forms which have
differentiated in separate regions or habitats are enabled to rejoin
each other. Intercrossing productive of obvious recombination
involving numerous characters may then result (p. 249), rather
than phenotypically continuous and simple intergradation. It is
in such cases that the criteria based on interbreeding and inter-
fertihty may both break down, and we must lay chief weight upon
degree of difference.*

* This must be unusually prevalent at the present time, partly due to the violent
changes of climate since the beginning of &e glacial period, partly to the post-
glacial rise of man to biological dominance. Owing to the activities of man,
many species and other groups which could otherwise have remained completely
isolated from each other, have met and hybridized, often with full fertility. This
may be due to indirect results of a changed ecological balance, to deforestation,
cultivation, or accidental transport, to deliberate introduction or deliberate
hybridization. The results of the sweeping rangc-changcs produced by fluctu-
ating climate must have been almost as extensive (see pp. 146, 358 .seq.).

l66 EVOLUTION : THE MODERN SYNTHESIS

At the opposite extreme are cases where related groups arc
entirely isolated from each other in nature, and normally never
cross, but yet show very httle morphological difference, in some
cases indeed none whatever {p. 296 Seq.). Other striking examples
are those where genetic isolation (p. 333) has occurred: here, more
attention must be paid to criteria such ^ geographical distribution,
but even so, doubtful cases will remain. Here convenience may
dictate the verdict: if it is impossible to distinguish forms on the
basis of preserved specimens, it is of dubious utility to give them
separate specific names.

In plants, polyploidy and asexual reproduction compheate the
picture. Most botanical authorities to-day would classify forms
differing solely in the number of chromosome-sets as “varieties”
or genetic subspecies, not as species, even if their inter-fertility
is lowered or absent. Similarly, authorities differ greatly as regards
their treatment of forms with purely asexual reproduction, like
the majority of the dandeHons {Taraxacum). Some wish to
designate every recognizable form as a species; this, however, if
pushed to its logical conclusion, would imply that each new
surviving mutation should be accorded specific rank. Turrill
(19386) suggests that for practical convenience a number of wcU-
marked forms should be recognized as species (agamospecies),
each comprising a number of separate asexual lines to be desig-
nated by the non-committal term biotype. Degree of mutual
resemblance and of distinctness from related populations here
become the main criteria of species, wliilc the idea of the inter-
breeding group has completely disappeared.

Where ecological divergence of two forms has occurred witliin
the same geographical area, spatially overlapping groups may be
kept from interbreeding by shght differences in mating habits,
food-preferences, or breeding dates, and so remain separate in
spite of the complete or almost complete absence of morpho-
logical differences. In many such cases again (c.g. in “biological”
or “physiological” races), the allocation of specific rank must be
a mere matter of opinion and convenience. FinaUy, where free
hybridization occurs, as in roses and brambles, the ordinary
categories of systcmatics, which arc adapted to divergent and

THE SPECIES problem: GEOGRAPHICAL SPECIATION 167

not to reticulate evolution, break down (p. 353). If the term
species is to be retained in such groups, it must be employed
mainly or merely on a basis of convenience.

Thus we must not expect too much of the term species. In the
first place, we must not expect a hard-and-fast definition, for
since most evolution is a gradual process, borderline cases must
occur. And in the second place, we must not expect a single or a
simple basis for definition, since species arise in many different
ways. On the other hand, if we ask whedier there is any greater
biological reality corresponding to the term species than to
higher systematic units such as genus, family, or order, we must
reply in the affirmative. Thus Dobzhansky (1937) is in entire
agreement on this point. As he writes (p. 306): “There is a smgle
systematic category which, in contrast to others, has withstood
all the changes in the nomenclature with an amazing tenacity.
... In most animal and plant groups, except in the so-called
difficult ones, the delimitation of species is subject to no dispute
at all.” And again (p. 309) : “Despite all the difficulties encoun-
tered in dassffying species in certain exceptional groups of
organisms, biologists have continued to feel that there is some-
thing about species that makes them more definite entities than
all other categories. W. Bateson has expressed this vague feeling
quite concisely: ‘Though we cannot stricdy define species, they
yet have properties which varieties have not, and . . . the distinc-
tion is not merely a matter of degree.’ ” Diver (1940) confirms
this from the angle of the ecologist, and Mayr (1940) from that
of the taxonomist: “It is quite amazing that in well-worked
groups there is hardly ever any doubt what is a species and
what is not’ ’ ; and investigation has steadily reduced the number of
cases wh'TC there is doubt as to the objective existence of specific
or subspecific groups. The number of “difficult” species in birds
is below I per cent. Again, of 755 birds listed as species by the
American Ornithological Union, only two are seriously disputed
(probably geographically isolated colour-phases). A further 94
are considered subspecies by “lumpers”; but even so, these are
objective natural groups. Allan (1940) agrees that species, in spite
of widespread hybridization, are “a reality of nature”.

i68

evolution:; the modern ■ synthesis

Wc cannot give any single reply such as that a species is an
interbreeding group completely isolated from breeding with
other similar groups; that would be an ovcr-simpHfication. But
we can say that living things, instead of showing continuous
intergradation, as might perhaps be expected a priori, tend to be
broken up into discontinuous group-units, distinguishable by
recognizable genetic differences in their characters, and that
practical convenience demands that these units, even though they
are of several types, originating in different ways and differing
in character and magnitude, be given specific names.

The scale on which this process of spcciation operates to intro-
duce discontinuity into the vital continuum, may be better
appreciated if we give a few figures concerning the approximate
number of species already described in different groups. Linnaeus
in the loth edition of the Systema Naturae described under 4,400
species of animals. This number has now been increased two-
hundrcd-fold. Hesse (1929), in a careful review, estimates the
total number of metazoan animal species recognized in 1928 as
between three-quarters of a million and slightly over one milHon.
Of this figure, the single class Insccta accounts for a minimum of

500,000 and a maximum of 750,000. The estimates for other
main groups are as follows: —

Sponges

Coelentcrates

Echinoderms

Annelids

Other Worms

Molluscoidea

Molluscs

Crustacea

Myriapods

Arachnids

Vertebrates

4,500

9,000

4,200

7,600

9,000

3>300

80.000- 104,000
15,500

8,100

28,000

40.000- 70,000

The variation in the estimates depends cliicfly on whether tlie
prmciplc of geographical replacement (p. 174) is adopted or not.
If adopted, the number of species is reduced, but many become

THE SPECIES PROBIEM: GEOGRAPHICAL SPECIATION 169

polytypic. The number is being steadily added to by the process
of discovery at an increasing rate which is already over 10,000
per annum in insects alone! (Smart, 1940). In the -well-worked
birds, however, Ma-yr (1940) estimates that under 100 undis-
covered species remain to be added to those already described.

Usually the identity of the discontinuous group and its delimit-
ation from other groups is preserved by interbreeding, though
in some cases, as in non-sexual forms like dandelions, the
delimitation is presumably achieved by selection-pressure. Some-
times the group is only potentially an interbreeding one; in other
cases the discontinuity which separates it from other groups is
not complete. In general, however, such discontinuous groups,
characterized by a particular area of distribution, and by discon-
tinuity in interbreeding or in degree of resemblance or in both,
do exist: and to them we can legitimately apply the term species.

An interesting analysis could be made of the general problem
of discontinuity in biological phenomena. Life is and must be
a continutam because of its basic process of self-reproduction: in
the perspective of time all Hving matter is continuous because
every fresh portion of it has been produced by pre-existing living
matter. However, discontinuities of .various sorts have been
introduced into the continuity. The .chief of these discontinuities
are those of the cell, the multicellular individual, the species, and
the ecological community. The last-named type of unit is very
instructive; in spite of continuous variation in environmental
factors, ecologicd communities are quite sharply separated, as
any one knows who has passed from the tree zone to the treeless
zone above it in mountain country (for a discussion on this
point, see Elton, 1927, Chap. 2). This type of discontinuity was a
constant source of preoccupation to Bateson (c.g. 1913, Chap. 8),
who also drew the attention of biologists to many others, such
as meristic variation.

Longlcy (1933) points out that if the quantitative relation found
by Willis (1922) between the frequency of genera in a given
group and the number of species they contain, can be gcncraHzcd
on a firm basis, it would provide independent evidence for the
biological reality of species.

170 EVOtUTION: THE MODERN SYNTHESIS

In all cases, the discontinuity, though fundamental, is never
absolute. Every biologist knows the Hmitations of the cell-theory
and the impossibility of giving a rigid definition of organic
individuahty, yet cells (Wilson, 1925) and individuals (Huxley,
1912) remain as essential biological units.

The same applies to species. Just as syncytia constitute an
exception to any rigid cell-doctrine, so large multiple inter-
bree^g groups, such as those found in willows or in man, form
exceptions to the usual rule of specific discontinuity. Intercellular
protoplasmic bridges find a parallel in the occasional exchange
of genes between otherwise discontinuous groups. The problem
of individuahty in colonial organisms with moderate division of
labour between the zooidsis matched by the problem of speciation
in groups intermediate between a Rassenkreis and an Artenkreis
(pp. 179 n., 407). Yet species, too, remain as essential biological
units.

Owing to the historical and philosophical association of the
word species, it might be thought desirable to employ some other
term in biological nomenclature. Owing to the fact that various
types of species exist, and that they exist in various degrees of
differentiatipn, it might be thought more scientific to replace
one by many technical terms. But species is hallowed by long
usage and so ingrained in practice that it would be virtually
impossible to replace it. Species, envisaged in this way as largely
or wholly discontinuous groups, are thus normally, though not
universally, realities of the biological scene: and it is our business
to sec what is known of the methods by which tliey originate
and by wliich their distinctness is maintained.

2. THE DIFFERENT MODES OF SPECIATION ; SUCCESSIONAt
SPECIES

It is logically obvious, on the postulate of evolution, that every
existing species must have originated from some pre-existing
species (sometimes, as we shall see, from more than one), but
equally clear on the basis of recent research that it may do so
in one of several quite different ways. A single species as a whole
may become transformed gradually to such an extent that it

THE SPBCIES problem: GEOGRAPHieAL SPECIATION 171

comes to merit a new specific name. Or it may separate, also
gradually, into two or more divergent lines whose divergence
eventually transcends the Kmits of specific distinction; sometimes
the separation into mutually infertile or otherwise distinct groups
may occur suddenly, but the subsequent divergence may yet ^
gradual. Or it may hybridize with another species and their
hybrid product may then, by chromosome-doubling, at one
bound constitute a new species, obviously distinct from the
outset: here, instead of one species diverging to form two, two
converge to form one. (It is possible that such sudden origins of
new species by means of chromosome or genome aberrations
may also occur without hybridization, from a single instead of a
dual origin.) Finally, in certain groups of plants, the minor
systematics are in an inextricable tangle, so that no two author-
ities agree even approximately as to die number of species
involved and their limitations; in these cases hybridization,
apparently involving many more than two forms, together
with back-crossing, recombination, chromosome-doubling, and
apomixis, appear to have been and stiU to be at work.

Thus species-formation may be continuous and successional;
continuous and divergent; abrupt and convergent; or what,
foEowit^ a recent writer (Turrill, 1936), we may caE reticulate,
dependent on repeated intercrossing between a number of lines
and thus both convergent and divergent at once.

We may thus classify the types of species-formation in varioxis
ways — ^whether they are gradual and continuous or sudden and
abrupt; whether they arc divergent or convergent; what kind
of isolation has been operative; what barriers to fertility have
been developed; and to what environmental factors, if any, the
process of species-formation is related.

We can distinguish four main kinds of factors which have
been decisive in bringing about the discontinuity leading to
speciation. These four factors arc time, space, function, and in-
trinsic mechanism. The four resultant modes of speciation are
transformation in geological time, geographical divergence,
ecological or adaptive divergence, and separation through genetic
accident. Thus, if we wish, we can distinguish four main kinds

X72 evolution: the modern synthesis

of species, the successional, the geograpliical, the ecological, and
the senetic. Naturally, die decisive agency m each case may be
assisted in a subsidiary way by the other agenaes. In geographical
speciation, for instance, there is normally an adaptive element,
while lapse of time and changes m genetic mechanism are mevit-
ably involved; but the factor of separation in space is primary

and decisive, that <
those of temporal and genetic

of adaptive functional change subsidiary, and
transformation merely conse-
quential and secondary. _ j _ i

Let us deal with these four modes of speciation in more deta .
The first three are always gradual, wliile the fourth may be,

though it is by no means always, abrupt. ’ . .

Our first major factor is time, producing successional speaatton.
In this process a given stock gradually changes its charactenstics,
so that forms meriting different specific and generic tides succeed
each other in time. Paleontology provides numerous evidences
of really gradual specific transformation; these have been pre-
served almost exclusively in aquatic animals such as amtnonitra
and other molluscs, sea-urcliins and other cchinoderms, though
also in a few land vertebrates such as the horses and titanotheres;
but similar changes must, it is clear, have been generally at work.
In some cases, as in the shift of. the mouth of the sea-urcto
Micraster, the change seems definitely to have been an adaptive
improvement— except possibly during the last phase, when some
authorities maintain that the original trend was prolonged
/^.t-Tirvrr^rirtirallv althouffh bv this tiuic uscless or dcleteirious

THE SPECIES problem: GEOGRAPHICAL SPECIATION I73

to form other species showing broadly parallel evolution; the
type is successively transformed. The subject of long-range
trends is of such importance that we deal with it in a separate
chapter (Chap. 9). Our analysis there will show that the great
majority of such trends are adaptive. Thus the main agency here
in producing successional speciation is selection, though it is
possible that orthogenesis (p. 504) may in some cases be at work.

It might accordingly be considered that time can never be the
primary factor in speciation. If orthogenesis is at work, the
primary factor would be genetic: it would be ecological when
the transformation is adaptive. It is true that time can never
operate alone to produce speciation, in the way that is possible
with alterations in genetic mechanism. Nevertheless it can rightly
be regarded as the m^or factor, or one of two major factors
working in combination, in all cases where we are considering
the transformation of a single stock. The transformation of the
horse stock from the three-toed into a one-toed type was un-
doubtedly in the main adaptive. Nevertheless, what separates
the forms along the single transforming line is time. It is the
length of time dut has elapsed between the genesis of one form
and of the next form meriting a separate name that has permitted
their specific distinction. This is because, in successional speciation,
we are dealing with stages in an evolutionary trend, not with
mere divergence in relation to peculiarities of the local environ-
ment or of the genetic constitution; and evolutionary trends are
normally long-continued, involving steady change in a single
direction over long periods of time.

To put it in another way, the distinction between two related
successional species is primarily a function of their separation
in time; while that between two related geographical species
is primarily .one of their separation in space ; that between
ecological species, of their divergence in mode of lifer and
that between genetic species, of changes in their genetic mechan-
ism. Of course here, as in every aspect of evolution, we are
dealing with processes of multiple causation. For instance,
successional adaptive trmsformation within a trend cannot pro-
ceed at all when a certain limit of specialization has been reached

174 evolution: THE MODERN SYNTHESIS

(p. 494); and its rate will be dependent on the stage of special-
bation reacted by the evolving type, as well as on environmental
conditions, in the same sort of way as the degree of ecological
speciation is dependent on predator-pressure (p. 324). In any
case, long-range evolutionary trends, considered as affecting
groups and manifested as adaptive radiation (p. 487), are pri-
marily affairs of ecological divergence. But each trend, considered
separately, is primarily an affair of successional transformation,
in which the successive forms owe there distinctness to the lapse
of time by which are separated their positions in the evolutionary
trend.

According to certain authorities, successional speciation often
proceeds, pardy or wholly, by discontinuous changes of small or
moderate extent. These are usually called “Mutations ofWaagen”
after the paleontologist who first drew attention to them. How-
ever, as Rensch {193 3<j) has pointed out, a much more probable
explanation of these is that a climatic or other environmental
change has produced a shift in geographical distribution, causing
a given stage in the stratigraphical sequence to be replaced by a
related subspecies or species which has difierentiated in another
region.

There are comparatively few cases in which environmental
conditions appear to have remained constant over a long period
in one area. But whether this be so or not, the change in the
fossds may be continuous, as with the sea-urchin M/criKter during
a considerable portion of the Cretaceous; in such cases we must
be dealing with intra-specific selection towards a higher degree
of adaptation. In the absence of evidence to the contrary, we are
probably right in thinkiag that successional transformation, in
the abundant species which alone can provide satisfactory fossil
series, is always or at least normally a gradual and continuous
process.

3. GEOGRAPHICAL REPLACEMENT: THE NATURE OP
SUBSPECIES

Next we come to cases in which divergence subsequent to some
type of isolation is the primary fact leading to the formation of

THE SPECIES problem: GEOGRAPHICAL SPECIATION 175

new species. The divergent splitting of species and genera must
clearly be postulated to have occurred on a large scale in the past,
if only to account for ihe rapid increase with geological time of
the number of types and taxonomic units in newly-evolved
groups, such as the orders of higher placental mammals. Most
of the divergence seen in such adaptive radiations of groups
(p. 489) is ecological, concerned with adaptation to different
environments and especially to different modes of life. It is, how-
ever, not easy to obtain from paleontology direct evidence of
divergence, since this demands good series in at least two separate
but crucial areas.

We shall return in a later chapter to ecological divergence as
illustrated from existing organisms. Here, however, we wdl
begin by dealing with geographical isolation, since a study of
geographical distribution reveals what are without question all
stages of geographical divergence. Furthermore, the data on this
subject are extensive, and have been subjected to thorough
analysis.

In aU cases, the basis on which we presume geographical
divergence, i.e., the evolution of a common ancestral form into
two or more different forms in different geographical areas, is
what has been called the principle of geographical replacement
(see, e.g., Rensch, 1929, 1933a). Under this we include the
numerous cases where closely-related but distinct forms (species
or subspecies) are found in different areas of the world’s surface,
but do not (with certain exceptions to be discussed later) overlap;
on the contrary, one replaces the other as we pass from one area
to another.

Such forms which replace each other geographically show aU
stages of diversity, from dubious and intergrading to sharply
defined subspecies,* and thence on to species and genera. As we
should expect, the percentage of groups which, though clearly
owing their origin to geographical differentiation, do not exhibit

* It is highly desirable to restrict the term subspecies to groups that are isolated
geographically or in other ways (e.g. physiologically) and also not to use the
term variety as synonymous with subspecies, but, if it be employed at all, to
restrict it to forms which occur together within the same geographical or other
group, as in polymorphic species (p. 99),

176 evolution: THE MODERN SYNTHESIS

Strict geographical replacement, but have overlapping areas of
distribution, is very low among subspecies (pp. 273, 291), but may
be considerable in higher taxonomic units which are intersterile
and have hadtimeforextensiverange-changes (pp.24i,243fF.,285).

Almost every group of organisms investigated reveals some
examples. Geographical divergence has been, perhaps, most
carefidly worked out in birds: our own avifauna provides
excellent examples, with die St. Kilda and the Shetland wren
{Troglodytes t. hirtensis and T t. zetlandicus), the British sub-
species of tit (Parus), jay (Garrulus), wagtail (Motacilla) and many
other forms, and the specific distinction of our red grouse,
Lagopus scotkus (seeWitherby, 1938-41). This last form, it should
be noted, has not only diverged specifically from the willow
grouse (L. lagopus) but has itself differentiated into a separate
subspecies in Ireland. Hartert’s classical work on palaearctic birds
(1903-35) illustrates the use of the systematic principle for a wide
range of forms, while Lynes’ exhaustive and elaborately illus-
trated study of the passerine genus Cistkola (1930) provides an
example of its application to a single type. In this single genus
he recognizes 40 species, with 154 subspecies. The genus is of
sedentary habits, so that the number of subspecies per species is
about 50 per cent higher than in related but migratory genera
(see p. 239) such as Phylloscopus (studied by Ticehurst, 1938)
or Sylvia.

Mammals, however, provide as good an array of examples.
We shall later refer to die dioroughly-investigated case of the
deermice (Peromyscus) (pp. 186, 188, etc.), but squirrels and other
rodents (e.g. Grinnell, 1922, on the kangaroo rats, Dipodomys).
antelopes, monkeys, and many other types behave in just the
same way. Insects, notably butterflies, have also received much
attention from this point of view. As an exhaustive study we may
refer to Warren’s monograph on Erebia, in which species,
many with marked subspeciation, arc recognized (Warren, 1936) ;
while Ellers (1936) has made an elaborate investigation of the
subspecies of a single species, the swallowtail Papilio machaon, and
Zarapkin (1934) aod End'rodi (1938) of the beetles Carahus
grofiulatus and Oryctes naskornis respectively. Zarapkin’s study is

THE SPECIES PEOBEEM: GEOGKAPHICAl SPEOATION 177

based on quantitative measurements of over loo characters.
We shall later also have occasion to refer to the geographical
variation of various mimetic and other butterflies, of moths,
beetles, reptiles, amphibia, snails. Crustacea (see Chevais, 1937)
and other animals.

Plants do not seem so prone to geographical subspedation as
animals, but a number are known which show the phenomenon.
Among plants an excellent example is Gentiana lutea, the large
yellow gentian. G. 1 . lutea, with free anthers, is widespread in
central Europe, while G. 1 . symphyandra, mainly distinguished
by its united anthers, is limited to the Balkans and their neigh-
bourhood: there is a slight amount of intergradation in an inter-
mediate zone. Gregor (193 8fl) has found geographical (as well as
ecological) differentiation in Plantago maritima (p. 223).

Fish are just as susceptible to the process as other animals. Even
deep-water spedes may show geographical differentiation, as has
been shown by Hubbs (1930), who finds that three forms of
Hymenocephalus striatissimus can be readily distir^uished, inter-
grading at the margins of their areas. The “races” of herrings
appear to be geographical subspedes, although the differences
between them have to be evaluated by biometrical as opposed
to ordinary taxonomic methods (see Schnakcnbeck, 1931); and
marine littoral types may be markedly differentiated into sub-
spedes. Again, according to Schilder and Schilder (1938) all the
165 spedes of living cowries {Cypraeidae) are divisible into
geographical subspedes, the number per species ranging from
two to seven or eight. Similarly the marine gastropod Turhinella
pirum shows well-defined geographical variation (HomeU, 1916).

Naturally, however, the process is better illustrated by types
with geographically discontinuous ranges, for instance, by the
d iff erentiation of the tree senedos in Afnca, where nearly every
high mountain has its own characteristic form (Fries, 1922). An
a dmi rable example from animals is provided by the difierent
forms of char {Salvelims) which inhabit various British and
Irish lakes. Where the char is still migratory, living in the sea and
ascending rivers to spawn, as in the northernmost parts of its
range, it is comparatively uniform; but when non-migratory

lyS evolution: THE MODERN SYNTHESIS

and landlocked in a lake, geographical diferentiation sets in.
Regan (1911) distinguishes fifteen forms in Great Britain and
keland. All these we should to-day classify as subspecies; for, as
Regan says in a later paper (1926), “Once you begin giving
specific names to lacustrine forms of char you never know
where to stop.” On the other hand, “if we were to exterminate
the char in our islands and on the continent, except in a dozen
selected lakes, we should have left a dozen wcH-marked forms
which it would be convenient to recognize as species.”

The biologically more or less irrelevant differences arising from
isolation arc in this case sometimes associated with certain adaptive
differences. For instance, the Loch Rannoch char, inhabiting a
very deep lake, has unusually large eyes; the habitual bottom-
feeders have blunter snouts and more rounded mouths. Thus, the
differentiation is partly geographical, partly ecological (see
p. 227). The whitefish (Coregonus) and the cisco {Leucichthys)
also show geographical differentiation in different lakes (see
Wortliington, 1940).* This lacustrine subspeciation of freshwater
fish can only date back to glacial times, when the lakes were
formed. The differences between trout and sea-trout and their
local differentiations are also of interest in this connection, though
too complex to summarize here.

Again, a large number of subspecies of rainbow trout are
restricted to single lakes or rivers in the western United States.
These last arc described by J. O. Snyder (1933) as separate species.
This is a result of his adopting the principle we have already
noted (p. 163), of employing lack of intergradation between
geographical forms as an absolute criterion of specific rank.
This, however, must lead to the pigeonholing of types which are
in point of fact at similar stages in the process of evolutionary
divergence, in different systematic categories. Absolute isolation
of groups will facilitate divergence: but that is a different point.

^ Some of the variants which have been given subspecific rank may prove to
be purely modificational forms. Thus Hile (1936), working on the North Amer-
ican cisco (Leucichthys artedi)^ finds that allometric growth, together with its
alteration owing to seasonal diiBfcrences in food-supply, etc., may induce form-
differences as great as some of those found in named subspecies. However, while
this points the need for more careful analysis, we can be certain that the majority
of the described forms have a genetic basis.

THE SPECIES PROBtEM: GEOGRAPHICAt SPECIATION I79

We shall here accordingly adopt the view, which is becoming
increasingly the basis of modern taxonomic practice, that forms
which replace each other geographically and the difierences
between which do not transcend those between intergrading
varieties, are (unless they are proved infertile by experiment)
best regarded as subspecies of a large species. The application of
this principle has much reduced the number of species recognized
in weE-investigated areas. Thus the twenty-six palaearctic forms
of wagtails {Motacilla) originally accorded specific rank are now
classified in four species with thirty subspecies: and instead of
nine species of palaearctic jays {Garrulus) one only is now recog-
nized (Rensch, 193 3«). The total number of bird species has been
rather more than halved by the apphcation of this principle
(Hesse, 1929).

When related and obviously “good” species replace each other
geographically we must conclude that the process of geographical
divergence has continued imtil the differences are of specific
magnitude. For groups of species related in this way Rcnsch
(193 3 fl) proposes the name o£ Artenkreis, which we may perhaps
in English call a geographical subgenus.*

The Artenkreis is a novel concept in systematics, but according
to Rensch it is a widespread fact of nature. Stresemarm (193 1)
applied the principle to the bird genus Zosterops (white-eyes).
In the genus as a whole he distinguishes twenty-two polytypic
species [Rassenkreise, or species with geographical subspecies) and
thirty monotypic species (without geographical differentiation).
Of these, he grouped fourteen polytypic and eighteen monotypic
species into six geographical subgenera {Artenhreise).

Similar phenomena are known in plants. Turrill (1929) gives
a number of examples of apparently good species from Crete
which are represented by closely allied species on the mainland.

* Some English-speaking authors trMslate Artenkreis by the term superspudes
or supraspedes* However, this shotild be restricted to intermediate cases, in wliich
the majority of the forms in a Kreis of groups showing geographical replacement
are clearly subspecies of a polytypic species, but a few have diverged further
until they are probably or certainly to be regarded as separate monotypic species.
It is in any case quite illegitimate to equate Rassenkreis and Artenkreis, as is done
by Schilder and Schilder (1938, p. 189), or, as they also do, to use the term
superspedes for polytypic species composed entirely of obvious subspecific groups.

l80 EVOtUTION: THE MODERN STiNTHESIS

Examples of “geographical species” from North America are
die Canada and the Oregon jays {Perisoreus canadensis
obscurus)-, and the mourning and MacGiUivray’s w^blers
(Ovoromis Philadelphia and O. iolmei). The two members of either
pair are both very similar, differing almost solely in details of
colour, and they inhabit different areas; they thus jointly constitute
an Artenkreis,

A similar exampk from Europe is that of the meadow and
red-throated pipits [Anthus pratensis and A. ceruinus). The coin-
mon and Carolina chickadees {Parus atricapillm and P. carolinensis)
are borderline cases: in some regions tlicy overlap without
intergrading, but in central New Jersey do interbreed (Chapman,

1924). . 1 , 1

Although in general, systematists who adopt the same prin-
ciples of classification will classify groups in the same way, there
arc bound to be dubious cases. A well-known example is that of
the Japanese pheasant, characterized by metallic green coloration.
This is by some authors classified as a separate species, Phcistauus
versicolor, but by others as a marked subspecies of the widely-
ranging common pheasant, P. colchicus (discussion in Rciisch,
193341, p. 28).

Numerous examples arc to be found of Russenktetse whose
extreme subspecies are so distinct that they would rightly be
classified as separate species if the intergrading connecting types
were not known. The char provide a case of this (p. 177). Among
the numerous further examples cited by Rcnsch (i 933 ^) we may
mention the Corcibus tHotiilis, Here the different subspecies,

in addition to large differences in size, shape, colour, ornamen-
tation, etc., are characterized by differences in copulatory organs,
which should prevent interbreeding. Some examples are known
when migration has brought extreme subspecies of a Rassenkreis
together and they prove not to interbreed. These arc cited on
pp. 243 seq.

A borderline case from plains is that of the bugles, Aji4ga
chamaepitys and A. chia (TurriU, 1934)* Here, cultivation ecotypes
seem to have been selected out and to have spread with agri-
culture to the N.W., until the extreme types have become

THE SEECIES PROBIEM : GEOGHAPHICAE SPECIATION 1 8 1

radically different from the ordinal Near-Eastern polymorphic
forms (p. 267).

The divergence of the marine fish fauna on either side of
Central America since the last union of the North and South
American Continents, probably in the early Miocene, provides
examples of a larger degree of divergence. In this case (Regan,
1906-8) the fish are hardly ever identical on the two coasts.
Usually a given form is represented by a pair of species, one
from either side, but sometimes the difierences are so slight that
the two forms can only be accorded subspecific ratak. It is of
considerable interest that although all the forms have been
separated for the same length of time, the degree of visible
divergence varies considerably from one species-pair to another.

The independent development of certain elements of the fauna
in large isolated lakes such as Baikal (see, e.g., Korotnefif, 1905-12)
and Tanganyika (see Yonge, I938«; Worthington, 1937) provides
examples of another kind of differentiation, in which certain
groups branch out into many types which have not evolved
elsewhere (pp. 324, 492). In such cases geographical isolation,
notably when combined with reduced selective pressure from
predators or competitors, opens the door to further dififerentiation
of the original type by means of ecological, especially ecobiotic,
divergence.

Different major groups, and different minor groups within
them, show differences in their proneness to diverge geographic-
ally; doubdess due to differences in their modes of life and their
environments; but it is clear that geographical divergence is a
general evolutionary phenomenon.

In wide-ranging species, different geographical races, or sub-
species as they are now generally called, may occur over a large
land-mass, intergradn^ genetic^y at the margins of their areas.
Where there are definite barriers, such as mountain ranges or
deserts, the intergradation may be absent, just as it ofien is with
island forms. AU stages in the restriction of gene-flow between
adjacent groups may of course be observed.

The house-wrens {Troglodytes) of South America, studied by
Chapman and Griscom (1924), provide a good example. Note-

, : \^^evolutio,n : ■ the ' mobesn ■ ■synthesis

worthy ia many subspecies of this group is the wide degree of
individual variation found. Distinctions between subspecies may
be based simply on alterations in the means of such varying
characters. Thus the subspecies often overlap in regard to thek
characters and are definable only on the basis of long series^ Steep
character-gradients (genoclines; p. 353) occur in the mixed zoties
along the borders of contiguous subspecific ranges.

Numerous cases of subspeciation in birds have been analysed
with great thoroughness. We refer later to A. H. Miller's work on
shrikes (p. 336). Here we may mention that of Swarth (1920)
and of Linsdale (1928) bn the fox sparrow (Passerella iliaca).
Linsdale studied the skeletal characters and found that these
show just as much variation (often in the form of geographical
character-gradients or clines; see p, 206) as do the plumage and
the general size. Some of these, e.g. those subserving flight, appear
to be adaptive: in every case the Sedentary or relatively sedentary
subspecies have smaller bones in the wings and pectoral gkdle
than do those with long migration routes. Linsdale could not
assign any adaptive significance to the considerable diifferences
in skull and bifl, though these may be ^'correlated characters"
(p. 206). No part of the skeleton was exempt from geographkai
variation, and there was a considerable though not complete
correlation between the geographical variation 01 skeletal and
of plumage characters.

In all these cases, the subspecies are relatively constant over
large areas, and the subspecific areas are separated by relatively
narrow intergrading zones. This state of ajffaks may be taken as
the ideal pattern of geographical subspeciation. Frequently, how-
ever, full details are unknown, and subspecific names are assigned
to forms from different areas simply because they are different.
In some cases, however, we know that there is no sharp delimi-
tation of subspecies by means of an intergrading zone, but only a
gradual delimitation; and, further, the “subspecies" itself may be
by no means constant, but merely represents the mean of many
differentiated local populations. This is so vuth some forms of
deermice {Peromyscus)^ as shown by Dice (1939), although in
other cases in the same genus the ideal condition is realized (see

THE SPECIES problem: GEOGRAPHICAL SPECIATION 1 83

below). Eventually it will be desirable to distinguish these two
types of intraspedfic differentiation by appropriate terminology.

The geographical variation in song demonstrated by PromptofF
(1930) in the chaffmch, Fringilla coelebs, appears to concern local
populations rather than being of true subspecific type (p. 308).

In other cases, excessive taxonomic zeal applied to insufficient
material in variable species- has resulted in individual varieties
being erroneously named as subspecies. This is well instanced by
the spotted hyena [Crocuta crocuta). No fewer than 19 forms of
this have been named, most of them originally as full species;
but die detailed study of Matthews (1939a) has shown that none
of these can be regarded as valid, though it is possible that two
or three geographical subspecies may be established kter if
sufficient material is forthcoming.

Warren (1937) draws attention to the fact that in the large
butterfly genus Erek'fl, different subspecies show, very different
degrees of variabflity. Facts of this sort clearly merit detailed
study in relation to ecology, selection, and population-size; we
need not at the moment accept Warren’s hypothesis of an
inherent recurrent cycle of variability.

Isolation of land forms by water, as occurs with groups inhabit-
ing islands, often leads to greater divergence, such subspecies being
unusually distinct (as with the St. Kilda wren) or having developed
into full species (as with the British red grouse). It is worth
mentioning that among the fifty-six species and subspecies of
mammals found in Scotland, more than half show a degree -of
difference meriting taxotiomic distinction from their continental
relatives — eight as full species and twenty-two as subspecies
(Ritchie, 1930).

The effect of complete isolation in promoting divergence is
especially clear in archipelagos where different islands often
harbour distinct forms (see p. 324). Examples of this fact occur
on the Galap^os (see, e.g., Swarth, 1931, 1934), and on the
Hawaiian Islands (e.g., in the birds known as sicklebiUs, Drepahi-
didae, p. 325; Lowe, 1936, discus'sions in A. Gulick, 1932, and
Mordvilko, 1937). Again, G. S. Miller (1909) points out that the
Malayan mouse-deer (Tragulus) exhibits only one form in the

i 84 evolution: the modern synthesis

wlioie of Sumatra and Borneo, whereas in the Rhio Linga
Archipelago off the tip of the Malay Peninsula, with i/i50th of
the land area and with less rather than more diversity of environ-
mental conditions, seven subspecies are to be distinguished. In a
subsequent section (p. 295) we deal with similar cases where the
barriers are of different nature. The high degree of differentiation
in these cases is doubtless due to the small size of the island
populations, which promotes “drift” and non-adaptive diver-
gence (cf. p. 200).

An interesting type of geographical divergence is one arising
out of the fact of dimorphism (or polymorphism). A species
which in most of its range exists in two (or more) distinct forms,
shows only one (or fewer) in certain restricted areas (see also
p. 104). For instance, the common squirrel [Sciurus vulgaris) of
the European continent exists in two forms, black and red, but
the British subspecies, besides showing certain quantitative pecuE-
arines, is monomorphic, without any blacks (p. 98). btresemann
(1923-26) refers to several analogous cases among birds. The
South American hawk Aaipiter ventralis, for instance, occurs in
its “normal” phase over the whole of its raige, in a Eghter and
reddish (phaeomelanic) phase over the whole range except for
a limited area, and in a dark (eumelanic) phase in a Emitcd area
only. An even more clear-cut case is tliat of Accipiter novae-
hoUandiae. Here a N-S dimorph-ratio cline in the proportions of
dark and white birds extends across Australasia; but on certain
islands to the extreme north only dark forms occur (Mayr,
1931-40, no. 41), and only white forms in Tasmama (p. 106;
Stresemann, 1923-6). In such cases we must assume that the
dark form is the original: it is accorduigly interesting to find tliat
in the snow goose Anser coerulescens the dark form is now restricted
to a very small area of the total range.

The phenomena of local melanic subspecies of Coereba, etc., are
referred to elsewhere (p. 203). A furdicr refinement of differ-
ential geographical dimorphism is seen in the cases where die pro-
portions of the two types vary regularly in space (dimorph-ratio
cEnes: see pp. 104, 161; the case of Accipiter novae-hollandiae falls
into this category, though the proportions here change abruptly).

THE SPECIES problem: GEOGRAPHICAL SPECIATION 185

The common red foxes present an interesting case (see Iljina,
1935). An Old World and a New World species are often distin-
guished {Vulpes vulpes and V.fulva), but most modem practice
regards them as highly difierentiated subspecies, or rather groups
of subspecies, since large numbers of minor subspecies of ordinary
type are recognizable. In addition, polymorphism exists in
almost every subspecies, due primarily to combinations of
three major distinct gene-pairs together with modifiers: the
polymorph ratios vary geographically. One major colour-
differentiation has a geographical basis : the true silver fox
depends on a gene found only in Canada. The black foxes of the
Old World are slightly different in appearance, and contain a
different gene: this, however, is also found in Alaska to the
exclusion of the Canadian “black” gene.

Complete geographical separation may also occur for eco-
logical reasons. Thus the moth Thera jmiperata feeds in the larval
stage entirely on juniper. Owing to the absence of juniper from
the English Midlands, the British forms of this species are restricted
to two separate areas, one in the north, the other in the south,
and as a result subspeciation has occurred.

Wherever experimaital analysis has been undertaken, it has
shown that the main differences between subspecies are of genetic
origin, and not due to environmental modification. Indeed, we
must lay down as a principle (although a decision may often
not be possible in practice) that non-genetic differences cannot
be accepted as a basis for subspecific distinction.

Recent analysis on neo-mendehan premises (see especially
Muller, 1940) has shown that complete or almost complete
geographical isolation (i.e. permitting no or negligible exchange
of genes with other groups) must be expected to lead, with the
lapse of time, both to morphological divergence and, usually
later, to physiological (genetic) discontinuity. This depends on
the fact that evolution proceeds by the incorporation of numerous
small mutational steps, and that each mutational step demands
buffering and adjustment through “internal adaptation”, by
mpatrs of modifiers (p. 67). The improbability that such
mutations will be identical in two isolated groups, even when

,l86 ' E¥0L'£JTI0N: the 'MGBERR STNTEE;SIS,^

environmental conditions are similar, is immense; and when the
two forms are subjected to different conditions, the divergence
from identity will be more rapid and more obvious. Similmy,
the internal adaptations of the germ-plasm will not be identical,
and disharmonies will arise leading to partial and eventually to
total reproductive disharmony between the two groups, either
by way of reduced mating, reduced fertility of Pi or Fi, or

rcduccdviability of Fi or later generations (p. 360). -r • 1

It is worth while recalling that under conditions of artifiaal
selection, isolation may frequently lead to divergence. Darwin
(1868) gives several examples of this phenomenon in Chapter 20
of his Variation of Animals and Plants under Domestication. The
most striking concerns two flocks of sheep, both bred ftom
Bakewell’s pure stock; after half a century, they had the
appearance of being quite distinct varieties”. la such cases, a form
of Sewafl Wright effect (p. 58) may operate, as well as uncon-
scious selection; but the effect of isolation is beyond question.
(See jdso D. S. Jordan, 1909, pp- 75 sefl-)

Analysis also shows that mere separation in space of two parts
of a population, even when biologically continuous, with free or
only slightly reduced gene-flow between them, will lead to
morphological diflferentiation when the environmental conditions
are sufficiently distinct in the two areas, since here divergent
selection win operate. The fact of relatively free gene-flow,
however, halts the process at the stage of partial biological dis-
continuity, resulting in intergrading subspecies (p* 209) •

Sumner (1932), following up the notable taxonomic study of
Osgood (1909), has made a detailed analysis of subspeciation in
Pcromysrw5. Perhaps the most striking case concerns three sub-
species of P. polionotus in Florida. P. p. poUonotus is dark in colour
and inhabits the interior, where the soil also is dark. P. p. lenro-
cephahs is extremely pale, and inhabits an island reef of pure
white quartz sand; and P. p. albifrons is somewhat pale, although
inhabiting beaches of the same white sand, but on the mainland.

Here wc undoubtedly have an example of the value of isolation
in counteracting the effect of migration and in permitting selec-
tion to act unchecked. In general Sumner finds it necessary to

oulcr aavantagcs wmcn may outweigh that ot cryptic resem-
blance. It may also be, of course, that certain groups have not
been long enough in their present habitats to permit the requisite
mutations to appear: with a low mutation-rate, mere chance
might make a great difFerence in the time needed to throw up
the required mutations.* On tliis hypothesis, selection of low
intensity is acting on all inhabitants of the white sand; but on the
mainland its effects arc partly counterbalanced by intermixture
with the dark inland race.

There is litdc doubt that this is part of the truth. On die other
liand, statistical investigation reveals that the mainland forms,
coastal and inland, not only intergrade but that they both show
a gradient of colour-rchange. This is moderate as the coast is left
for die interior. Then, about forty miles inland, follows a narrow
strip a few miles wide where the gradient is very steep, and finally
a region where the gradient is very gradual indeed. The zone of
rapid change must be regarded as die boundary between the
two subspecies; interestingly, it does not occur at the same
place as docs a geological change involving a darkening of
soil-colour.

To account for these and similar facts in other races, Sumner
assumes that each race has a main area, and is subject to large
periodic fluctuations in abundance,, such as Elton has shown to
occur in most small mammals. In periods of over-population,
migration will be initiated (Elton, 1930), and will presumably

* A case bearing on this point is that of the local population of houscniicc
{Mus musculus) on a small sandy island in Ireland, studied by Jameson (189B).
The average coloration of the population was considerably lighter than normal,
but with great variability: the paler animals* colour matched the sandy back-
ground. Predator-pressure was intense owing t<> the lack of cover. From a
carcflil study of map.s Jameson estimated that the island could not have been
isolated for more than 100 to 125 years. Meinertzhagen (1919) mentions that the
introduced goldfinch (Caniuelts carduvUs) in the Bermudas now merits sub-
specfic rank, and that die introduced starling (Sturnus tmlj^aris) in Cape Colony
lias already lost the migratory habits though this may be only a modificntkni

l88 evolution: THE MODERN SYNTHESIS

be most intense in directions away from the mam centre of the
population, or as Sumner puts it, in the direction of a falling
gradient of population-pressure. Two contiguous subspecies will
thus be pressing against each other like two inflated rubber bags,
and the boundary between the two will shift according to the
relative degree of population-pressure. Just at the boundary,
migration will be producing intercrossing. Owing to the principle
of harmoniously-stabilized gene-complexes, the zones of inter-
crossing will remain narrow even when their location is shifted
(p. 209). It is thus quite possible that a subspecies which originally
differentiated in relation to some particular area will spread over
a much larger area. Thus the type of adaptation which we actually
find, namely, a rough general correspondence between adaptive
characters and habitat but with numerous exceptions of detail,
is to be expected.

It is worth pointing out that such zones of rapid change with
intermixture have been found in numerous other cases where sub-
specific distribution has been thoroughly investigated— e.g. with
numerous types of birds in Lower California (Grinnell, 1928), as
well as the wrens and sparrows already mentioned (p. 182).

In general Sumner’s hypothesis seems to fit many of the facts
very well. As further consequence, it may turn out that certain
subspecies occupying a large and diversified area represent the
sum of a number of originally separate races which have united
by migration: if so, in some cases subspecies may be of poly-

phyleticorigin,asregards their evolution within the species (p.291).

Thus it seems clear that some characteristics of the subspecies
of Peromyscus, such as coloration in some form, must be directly
adaptive (see Dice and Blossoip, 1937)- Others, however, such
as absolute and relative tail-length, have no obvious adaptive
value. They may be accidental by-products of isolation; or they
may be correlated with less obvious but deeper-seated physio-
logical adaptations. Yocom and Huestis, for instance (1928),
have shown that a coastal and a desert subspecies of P. maniculatus
differ in important characters of their thyroids, the coastal
variety having less active glands with greater accumulations of
secretion. These differences in glandular make-up are quite

THE SPECIES problem: geographicai. speciahon 189

possibly the direct cause of the differences in colour, which would
then be non-adaptive “correlated characters”. Sumner (op. cit.,
p. 98) states that later work shows it to be “just as easy to distin-
guish these two subspecies by the thyroids as by the pelages”.
He also finds that certain subspecies are distinguished by constant
differences in general activity. Griimell (1928), on the basis of
great experience, believes that the differentiation of subspecies
(in birds) is basically adaptive.

In some cases the subspecies arc polymorphic, e.g. in P. mani-
culatus hlandus, buff, grey, and all intermediate types are found
(see Dice, 1933a). Here all the colours cannot well be adaptive,
but some selective balance must be operative (p. 97).

However, the principle of the correlation of visible and appar-
ently non-adaptive characters with deep-seated adaptive properties
is undoubtedly widespread. Dewar and Finn (1909, p. 357) cite
a case from domestic pigs and sheep in America. The light-
coloured breeds are poisoned by various plants, while the dark
breeds are immune. Black pigs, for instance, are not injured by
eating the paint-root Lac/iMa«tAes.

A classical case is that of the rubrinems mutant of the evening
primrose Oenothera lamarckiana. This, as its name impUcs, is
distinguished by the red colour of the veins on its leaves and
elsewhere. But it abo shows accelerated pollen-tube growth and
increased resistance to cold.

Haldane (1933a, 1932c) draws attention to other cases in which
gcnetical experiment has proved the dependence of two or more
very distinct phenotypic characters on a single gene. Thus in
stocks (Matthiola incana) hairiness depends not only on two special
genes for hairiness, but also on a gene for colour in the flowers
(Saunders, 1930). Thus selection for hairiness would, in certain
heterozygous populations, automatically ehminate white-flowered
plants. A still simpler case is that of the gene increasing the size
of the central “eye” in the flowers of Primula sinensis: this abo
reduces the style-length in genetically long-styled plants, pro-
ducing a homostylc in place of a ^*pin.” flower. Thus the normal
arrangement for bringing about cross-fertilization can only
operate in small-eyed flowers. Again, many genes in Drosophila

evolution: THE MODERN SYNTHESIS

produce multiple effects, e.g. on bristles and wings. We have
already mentioned (p. 8o) the effect of the white series of eye-
colour alleles on certain internal organs. Haldane also recalls the
fact that apparendy irrelevant genes may restore physiological
balance and viability. We have given examples of this in
Chap. 3 (pp. 68 scq.). A further probable case is the increased
viability of “arc”-winged mutants when an axillary spot is added.
Sometimes the correlated characters are merely modifications,
which appear only in certain environments. The best example
of this is the frizde fowl (p. ii8), in which the thyroid is
enlarged as well as the feathers altered. Investigation shows,
however, that the thyroid effect only occurs in cool climates, and
is a reaction to the excessive heat-loss caused by the inadequate
feathering, which is the only direct genetic effect. (See also p. 533).

Whenever a genetically-determined cline or character-gradient
(p. 206) exists in visible characters, even if these are apparently
non-adaptive, and is correlated with a gradient in the environ-
ment, we are justified in assuming a further correlation between
the visible characters and adaptive physiological properties. In
aU such cases, the onus of proof is on those who would deny the
direct or indirect adaptiveness of the graded characters.

It is interesting to note that Smnner began his laborious investi-
gations with a bias in favour of the subspecific characters of deer-
mice being due to the hereditary fixation of the direct efects of
the environment, and against the view that they were determined
by mendelian genes. In the course of time, however, the facts
induced him to abandon this position, and he now believes that
natural selection has been an important agency in establishing
subspecific differences, and that most subspecific characters are
not only “genetically determined” but mendelian.*

The long-tailed field-mouse, Apodemus, GBs the same eco-
logical niche in the old world that Peromyscus docs in the new.
Though it belongs to the murine section of the mouse family,
as opposed to the cricetine, it is very similar to Peromyscus in

* So recently as 1921, it was possible for distinguished ornithologists to
express the view that most subspecific characters in birds were mere environmental
modifications (Lowe and Mackworth-Praed, 1921). To-day all would agree that
the great majority are genetically determined.

THE SPECIES PROBIEM: GEOGRAPHICAl SPECIATION Ipl

appearance, and also shows marked geographic variation. Mr.
Hinton tells me that he believes this genus would show a very
similar correlation of type of geographic variation with climate
and sod, but the detailed analysis has not yet been made.

In general, it appears that some at least of the distinctions
between subspecies are adaptive, but, when not obviously cryptic,
in relation to local background, are usually of a general nature,
in some relation to cHmate. Such a relation may be direct, as m
cases of differing temperature-resistance, or indirect, as in the
greater prevalence of migratory habit in bird subspecies from
higher latitudes. Goldschmidt, in an exhaustive series of studies
(summary and references in Goldschmidt, 1934), has shown that
trivial and apparently useless differences between geographical
races of the gipsy-moth Lymantria dispar are accompanied by
physiological and reproductive differences of great significance
in relation to climatic conditions (p. 436).

Timofeeflf-Ressovsky {1935) has shown that the widespread
population of Drosophila funehris in Europe, though showing no
visible subspecies, is geographically differentiated in regard to
temperature-resistance. The adaptation is a deHcate one. Thus
the Western European strains are especially susceptible, the
Russian and Siberian ones especially resistant, to the extremes both
of heat and of cold, while those from the Mediterranean are
resistant to heat but susceptible to cold.

A curious case is that of the chat Oenanthe lugens. In Egypt
both sexes are alike, with conspicuous coloration; but in the
Algerian subspecies, though the males are very similar, the
females are of a sandy colour. Here there seems to be a local
protective adaptation of the female only. This case from birds is
paralleled by various butterflies, notably the swallow-tail Papilio
dardanus. In the subspecies inhabiting Madagascar, both sexes
are alike, resembling the male of the other subspecies. In those
from the Afiican mainland, however, the females are nearly
always mimetic, often polymorphically so (p. 123), except in a
few special areas (Eltringham, 1910). It would appear that where
the struggle for existence is more intense, the female, with her
greater biological value, is often protected before the male (as

192 evolution: the modern synthesis

undoubteclly occurs in many birds: Huxley, 1938c), altlmugh it
would also appear likely that selection is acting to keep the male
uniform, so that any stimulative or recognitional function
exerted by his coloration in regard to mating may be pressed,
unimpaired by any break-up into several forms of different

The view that subspeciation is in any large measure adaptive
is not universally held. Only recently D. M. S. Watson (Watson
and others, 1936) wrote, ‘It is probable that the differences
between geographical races (which have only a statistical mean^mg)
have no adaptive significance,” a statement which is only a little
less sweeping than his earlier one: “I do not know of a single
case in which it has been shown that the differences which
separate two races of a mammalian species from one another
have the slightest adaptive s^nificancc.” Quite apart from me
Statement concerning the merely statistical nature or the dis-
tinction between subspecies, which is by no means always or
even usually true, this dictum would not correspond with the
consensus of biological opinion (see, for instance, Grinncll, 1928).
It is unlikely that mammals and birds would differ in this respect,
and apart from the mammalian case of Peromyscus, wc have that
of the crested larks {Galerida) and other birds of semi^esert
country in which Meinertzhagen (19^^) has shown a strong
correlation, undoubtedly protective, between colour of plumage
and colour of soil. Moreau (1930) finds similar phenomena in
some Egyptian bird subspecies. Again, in the African buffaloes,
the gradual reduction of body-size and of relative horn-size
shown by C. Christy (1929) to occur with increased density of
forest is clearly adaptive. It is noteworthy in this case that skull-
size is little affected: the difficulty of moving rapidly through
dense forest would depend much more on body- and horn-size
than on this.

In the African squirrel Heliosciurus gambianus, Ingoldby (19^7)
has shown a marked correlation between climatic conditions and
visible characters, the forest forms being saturated in colour and
larger, the savannah forms pale and smaller, and with aU grada-
tions between. The adaptive nature of these particular characters

THE SPECIES problem: GEOGRAPHICAL SPECIATION 193

is not apparent, but the close correlation of environmental and
character gradients makes it impossible to believe that the
characters are the result of chance: they are presumably, when
not mere modifications, correlated with non-apparent adaptations
(pp. 63, 206).

In this case, by the way, Ingoldby maintains that many forms
do not have definite geographical areas, but recur sporadically as
the climatic conditions dictate. This would be contrary to our
experience in cases like that of Peromyscus which have been very
thoroughly worked out, and it is likely that in the squirrels, in
addition to these more obvious characters of pigmentation and
size, others will be detected which will enable a truly geographical
(as well as an ecological climatic) distribution to be worked out.
The squirrels of the genus Callosciurm of Lower Burma (Thomas
andWroughton, 1916), which also show great colour-variation,
do conform to such a scheme, certain species being sharply separ-
ated by the Chindwin River. Curiously enough, differentiation
has been much more active on the east than on the west bank of
the river; the differentiation, however, appears, to take the form
of a number of clincs instead of well-defined subspecies (p. 219).

The increase in wing-length of open-country subspecies when
contrasted with forest forms of the same species, as foimd by
G. L. Bates (193 1) in West African birds, is a clear case of adaptive
difference. Similar adaptive differences in wings and tail have
been found in the subspecies of fox-sparrows and shrikes by
Linsdale (i928)andA.H.Miller (i93i)respectively (pp.182,236).

Bates also found various cases of dines or character-gradients
correlated with environmental gradients. We shall give further
examples of such character-gradients later (p. 206). As already
pointed out, these, in so far as they are genetic, must be cither
directly adaptive or correlated with some internal physiological
adaptation.

Undoubtedly genetic accident plays a part in determining the
characters of subspecies; but its role will be most important in
small and entirely isolated groups, whereas with groups showing
continuous distribution over larger areas it wiU tend to be over-
shadowed by the influence of selection. Wc refer elsewhere to

194

evolution: the modern synthesis

some examples of the Sewall Wright effect, or drift (pp. 200. 242).
From the wealth of facts available, we ate a couple more here.
Murphy and Chapin (1929) find two subspecies of goldcrest
(Remlus reguhs) in the Azores, one generally distributed except
on die island of San Miguel, where alone the second form exists.
Using elaborate genetic analysis, Dobzhansky (i 939 ?) finds that
Drosophila pseudoobscura has smaller effective breedmg popu-
lations in the north of its range than m Mexico and Guatemala,
and tliat this has led to-the northern populations showmg reduced

genetic heterogeneity (see pp. 60, 371-2)- . . r

We often know the approximate date at which isolation ot an
island has occurred, and can see that broadly speaking, ^nough
with a considerable amount of variation (pp. 200, 324; and below),
the degree of divergence is proportional to the tune that has
since elapsed, as well as to the effectiveness of the isolation. It is
thus a perfectly legitimate deduction that geographical vanatton
of the type we have been considering provides us with a criMS-
section of a temporal process and that isolational divergence has
been constantly operative throughout evolution, as an agency
promoting minor systematic diversity. Moreau (1930) on the
basis of the known facts concerning post-glaaal changes m
geology and climate, has discussed the age of various Egyptian
subspecies of birds. He finds that several cannot be older than
10,000 years, while one or two must have an age of only 5,000
years or slighdy less. He is inclined to put 5,000 years as the
normal minimum time for distinct subspeciation, on the ground
that lower Mesopotamia, where , the land has only come into
existence during the last 5,000 years or so, shows no endemic
passerine subspecies, and very few others. Approximately similar
periods would hold for the subspecies of birds and mammals
found on islands off Scotland, which can only have been colonized
in post-glacial times; the same apphes to die differentiated races
of frogs (p. 235). However, goldfinches [Carduelis carduelis)
introduced, apparendy recently, into Bermuda arc now appreci-
, ably darker (Kennedy, 1913). and die facts concerning rats and
mice (pp. i87n, 257) show that subspecific differentiation may
sometimes occur much more quickly.

THE SPECIES problem: GEOGRAPHICAL SPEaATION 195

In particular, the Faeroe house-mouse, Mus musculus faeroettsis,
which was introduced into the islands not much more than 250
years ago, is now so distinct that certain modem authorities have
assigned full specific status to it (see Evans and Vevers, 1938).

Rabbits have been isolated on Skokholm island (S. Wales)
for about six centuries. They now average 120 g. below main-
land weight, and are blacker above. This is moderately rapid
differentiation, though the result does not yet merit subspecific
naming (Lockley, 1940).

Temperature must influence the rate of differentiation to a
certain extent. Thus Hubbs (i94oii) finds that the subspeciation
and speciation of fish populations isolated by the desiccation of
the American desert is more rapid in warm springs than in pools
at normal temperature.

Accidental “drift” in small populations may, of course, rapidly
bring about sHght differentiation. Thus a colony of the heath
fritiUary butterfly {Melitaea deliberately introduced into

Essex within the present century, is already noticeably smaller
and darker than the Kent strain, firom which it was derived
(Stovin, 1937). Harrison (1920^) showed that in the moth Oporabia
autumnata two local populations inhabiting ecologically distinct
woodlands, became quite distinct in size, colour, and certain
physiological characters in a very short period of years. Salomon-
sen (1938) gives evidence to show that the white-headed form
of the barbet Lybius torquatus, which is localized to the east and
south of Lake Nyasa, has spread westwards in the last forty
yean. This form (originally described as L. zomhae) appears to
have originated by at least two mutational steps, as piok-headed
types, intermediate in various degree, are also found. At Somba
in the eighteen-nineties about half the population still had light
red heads, though no dark-red birds were present. In 1933,
however, no Hght reds occurred, and, apart from an occasional
hght pink, all the birds were white. Salomonsen considers tliis
as evidence of the transformatioii of a whole population by the
spread of mutant genes, though Meise (1938, p. 68) thinks it
represents the shifting of a zone of hybridization between two
well-marked subspecies as a result of population-pressure.

EVOLUTION! THE MODERN SYNTHESIS

An extremely interesting point is brought out by Swarth
(1920, p. 106, map), concerning the migratory habits of the
fox-^sparrow, Passerelln iliacc. The uncileschensis group of sub-
species breeds along the north-west coast of North America.
Five well-marked subspecies succeed each other as we pass north-
wards along the coast. The southernmost {P. i. fuliginosa) is to
all intents and purposes a resident. The others are migratory,
but in their migrations play leap-frog over the intervening forms.
Thus No. 2, reckoning in breeding range from south to north,
winters just south of No. i fuliginosa). No. 3 breeds north of
No. 2, but winters to the south of No. 2s winter range; and
Nos. 4, 5, and 6, whose breeding ranges succeed each other to
the north-west of No. 3’s, winter together in the extreme south

of the winter range of the group.

The obvious explanation is that the resident subspecies persisted
in its present range throughout the last glacial period. As the ice
receded, No. 2 invaded new breeding territory, but was forced
to winter south of the already occupied range of No. i : Nos. 3 *
4, 5, and 6 repeated the process, but the last three were crowded
together into a single winter area close to the southern limit to
which the species is adapted. If so, the differentiation of the
uorthemmost subspecies must have been effected during the last
10,000 years or less.

In numerous instances, forms meriting classification as species
are found geographically isolated from their nearest relatives, and
must be presumed to have owed their origin to an extension of
the divergence that leads to subspeciation. Naturally, they will
tend to occur more often where the isolation is more thorough.
We have mentioned the red grouse of Britain, Lagopus scoticus,
whose nearest relative is the willow grouse of Scandinavia, L.
lagopus;. it should be recalled that one of the most important
specific distinctions in this case is adaptive, namely the willow
grouse’s winter change of plumage to white, and the absence of
this feature in the less extreme climate of Britain. The ptarmigan
{Lagopus mutus), which is a bird of higher latitudes and altitudes,
becomes white in winter in both -regions.

Another case is that of the snail, Truncatellina britaiinica, closely

THE SPECIES problem: GEOGRAPHICAr. SPECIATTON 197

allied to and doubtless derived from the continental T. rivierana.
Excellent examples from plants are found in the European
Gesneriaceae, notably in the genus Ramondia — e.g. R. serhka
from Serbia and neighbouring areas, R. Mdreichii from Thessaly,
and R. pyremica from the Pyrenees. These would thus constitute
an Artenkreis (p. lyp)* Numerous other plant cases may be found
in boob such as Willis’s and Area (1922) or in phytogeo-
graphical works such as Turrill’s Plant Life of the Balkan Peninsula
(1929). in some cases the geographical variation appears adaptive,
but in others, as for instance the marked fruit variation in Clypeola
jonthlaspi, no adaptive interpretation can be given (TurriB, in
Watson and others, 1936).

We must, however, mention the view of Goldschmidt (1932,
193 5> 1940) that the formation of geographical subspecies a nd
that of true species are wholly distinct processes. The former,
according to him, involves only quantitative modifications of a
basic genetic pattern, while the latter involves the formation of a
new inherent pattern. This production of a new type of equi-
librium, he is inclined to think, is achieved abrupdy. While this
may apply in some cases (though there is no direct evidence for
it as yet) it would appear impossible to deny that the divergence
which produces subspecies does in fact often lead on to the pro-
duction of species, more especially since the distinction between
subspecies and species is not (and indeed ca nn ot be) a sharp or
universally agreed one (see p. 456).

A rather different type of geographical subspecies may occur
in rare species. Rare species will not normally be spread more or
less continuously over a wide area, but will often exist in pockets
here and there, whether because they have not been able to
spread or because they are in process of being ousted by other
species. In such cases there will already be considerable isolation
of groups. Thus any selective agencies can work without being
counteracted; further, even new non-adaptive mutations and
recombinations can establish themselves much more readily in
a small group (Wright, 1931, 1932, 1940). Indeed, in certain
cases, the coune ofevolution may possibly be determined by mu-
tation-pressure (Wright, 1940). We may distinguish these as heal

198 evolution: the modern synthesis

subspecies from the areal or regiowaf subspecies of abundant
species, and it may be expected that they will owe their diver-
gence more to chance recombination and less to selection; their
distinctions will tend to be trivial and useless rather tlian adaptive.
Bateson (1913) gives numerous examples of both types.

We have already mentioned the case of the fern, Nephrodiutn
spinulowm {p. 33). A sHghdy different example, since the range
covered is greater, is afforded by the rare moth, Rhyacia alpicola.
This occurs only in small restricted areas, in each of which
considerable differentiation has occurred. One subspecies exists
in Lapland, another in Ireland and Scotland, a third in the
Shetlands, and a fourth in the Carpathians.

Some of the local groups of die genus Sorbus (service-trees,
etc.) seem to be local subspecies in diis sense (Wilmott, 1934).
One wcU-marked form, for instance, occurs only in ihe Avon
Gorge, another in the Wye Valley, another only near Minehcad,
and so on.

An interesting case of local variation, presumably mutative, is
given by Salaman (Watson and others, 1936). The wild potato-
like plant. Solatium demissum, in one part of its range is genetically
resistant to common bUght {Phytophthora infestans), but is sus-
ceptible in another area. The resistant strain occurs in a region
where blight is not found, so that we have here an example of
potential pre-adaptation (pp. 450 seq.).

The jimson-weed. Datura stramonium, shows geographical
differentiation in regard to its chromosomal structure, various
“prime types” produced by segmental interchange (p. 90) having
a well-defined distribution (Blakeslee, Bergner and Avery, 1937).
It » possible that they may all originally have shown gcograpliical
K placement, but the fact that the species is a rcachly-distributcd
weed has confused the distribution; in any case, some regions
now contain two or more cliroraosomal races (p. 329).

Some abundant species show highly locahzcd varieties which
may also be called local subspecies — for instance the common
thyme, Thymus serpyllum, and the sea-campion, -Silme maritima.
The reason for such localized differentiation in these cases is
obscure, as is the reason for the local existence of obvious single-

■ ■ THE SPECIES PROBLEM : GEOGRAPHICAL SPECI AXIOM 1 99

gene mutants, sucli as wliite-flowered plants, in patches or in
isolated valeys.

Among birds where a presumed large mutation h'as become
diagnostic of a taxonomic form is B. bmnneinuchus^ a wide-
ranging species of sedentary habits (Chapman, 1923). B. inor-
natus differs only in its slightly smaller size, and in the absence,
of the black breast-band characteristic of the former. Its habits
and ecological preferences appear to be the same. It exists in
a rather isolated vaUey in the centre of the range of B. hrubbeim--
chus and there replaces its relative. There can be little doubt that
it represents a geographical form (probably a subspecies, not a
species, ho wever) of which the chief characteristic is the presum-
ably abrupt (mutational) loss of the breast-band. In B. assimiUs
sporadic individuals of one well-marked subspecies show
characters diagnostic of odier subspecies or species: sec also
Chapman (1927). In the Papuo-Mclanesian bird Formenkreis
Lalage aurea (Mayr and Ripley, 1941, Amer. Mus. Novit
no. 1116) barred plumage of underparts has been independently
lost at least five times, and independent mutation seems to have
occurred in other clear-cut characters such as eyestripe.

The buttercup Ranunculus allegheniensis appears to have
differentiated in a way essentially similar to Buarremon inornatus
(Gates, 1916), since it is found abundantly in a comparatively
small area within the range of the widely-distributed J?. ahortivus^
which it there replaces and from which it differs by a few minor
characters and one striking, probably mutational distinction in
the shape of its achenes.

In passing, a curious case of geographical difference in Dro-
sophila may be mentioned Timofeeff-Ressovsky (1932^1) finds
that the wild-type alleles of the white-eye series in European and
American D. melanogasier are not identical. The American allele
mutates nearly double as often with the same dose of X-rays,
and gives a higher proportion of full white genes among its
mutations. Here we have a geographical difference in intrinsic
capacity to vary.

The proof given by Wright, that non-adaptive differentiation
will occur in small populations owing to “drift’’, or the cliancc

200 evolution: Tin; modern SYNTHr5SIS

fixation of some new mutation or recombination, is one of die
most important results of mathematical analysis applied to the
facts of nco-mendelism. It gives accident as well as adaptation
a place in evolution, and at one stroke explains many facts which
puzzled earlier selectionists, notably the much greater degree of
divergence shown by island tlian mainland forms, by forms in
isolated lakes than in continuous river-systems. We have given
numerous examples of such phenomena. Turcsson (1927) uses
the terra “seclusion types” for such forms in plants. Recently
Kramer and Mertens (193 8<i) have provided a quantitative
demonstration of the principle, in their work on Adriatic lizards
{Lacerta sicula). Surveys were made of the lizard population of
a number of islands, and the degree of their divergence from the
uniform mainland type was determined on an arbitrary scale. At
the same time, the depth of water between each island and the
mainland was noted; this can be regarded as a measure of the time
during wliich the population has been isolated, since the islands
have been formed by subsidence. Further, the area of the island
can be used as a measure of population-size. It was found that
degree of divergence showed definite partial correlations, both
directly witli length of isolation, and inversely with size of island.
The table opposite, based on Kramer and Merten’s data,
brings out the point. Island-size is denoted on a logarithmic
scale, subdivided in its lowest part, since the intensity of the
Sewall Wright effect increases rapidly with decreasing size of
population.

0 indicates identity with the mainland form, 4 the greatest
divergence found. The least divergence is ^own on large islands,
the greatest on rather small islands after long separation; very
small islands may show considerable divergence after very short
separation (see also pp. 187 n., 195).

In the white-eyes of the wide-ranging bird genus Zostcrops
(sec p. 179 and Stresemann, 1931) the degree of differentiation
of island species (or subspecies) appears to be correlated with a
considerable number of factors — (a) directly; with (i) the age
of the island, (2) the inherent mutability of the stock; (b) in-
versely; with (3) die size of the island, (4) the predator-pressure

THE SPECIES problem: GEOGRAPHICAL SPECIATION 201

and degree of competition from related forms, (5) the degree
of migration.

• It should be noted that if a population is subjected to cyclical
fluctuations of abundance, the determining factor is the size of
the minimum effective breeding population. In extremely small
populations, tibe SewaU Wright effect may even fix deleterious
mutations, and so result in extinction. Various of the cases where
protection of the remnant of a once-abundant species have failed

Differentiatton in Island Lizards

area

(arbitrary

0-^5

6-12

12-18

i 8~24

24-30

30-36

units) ■
0*5 ,

0*5

. I

1-5

If 2

1,2

2 . 3 J,

3h3

5--I0

lO-IOO

IOO«I,OCX)

to prevent further decline and eventual extinction are probably
due to this cause. The best-documented example is the extinction
of the subspecies of the prairie chicken known as the heath hen
{Tympanuchus c. cupido), in spite of the most elaborate protective
measures (Gross, 1928). Conservationists should take note of
this. If their efforts to save a dwindling remnant of a species do
not bring about a rapid increase of numbers, they arc likely to
be in vain: early action is essential.

Geographical differentiation may be carried far beyond the
stage of broad subspecies to a high pitch of local detail. When
small populations are completely or almost completely isolated

202 1-VOLUTION: THE MODEUN SYNTHESIS

from each other, almost every such population may develop
its own distinctive characters. This is so, for instance, with
Partiila and other snails of the Pacific Islands (p. 232), with the
insular lizard populations of the Adriatic and elsewhere (p. 200),
with certain flightless grasshoppers of arid regions (Uvarov, in
verbis), etc. Hubbs (1940^) finds marked diftcrentiation in quite
small populations of fireshwater fish (a few hundred individuals),
isolated in pools as a result of the desiccation of the American
desert. Differentiation is then often apparently non-adaptive.
Frequendy the differences, though definite, arc not considered
by experienced taxonomists to merit a subspecific name: e.g.
some of the insular lizards; various insular birds, such as the
Fair Isle wren (see discussion in Huxley, 1939a, and in J. Fisher,
19390) and others cited by Mayr (1931-38), etc. But in other
cases, as in the grasshoppers just mentioned, the differentiation
is considerable, and the only difference from ordinary subspecies
lies in the small size of the groups. If a general term is needed
for such cases, microstdbspecies is perhaps preferable to that of
micro-race proposed by Dobzhansky (1937). Goldschmidt (1940)
uses the rather awkward term “subsubspecics” .

Microsubspccics are preferably not to be given names subject
to the international rules, since diis would complicate the nomen-
clature unnecessarily.

Even finer differentiation may occur. Thus Diver (1939) in
the snail Cepaea finds that the proportions of the various types of
colour and banding vary from colony to colony, almost always
in an arbitrary, non-graded way; he also gives similar examples
from other land and freshwater moEuscs. Lloyd (1912) and
Hagedoorn and Hagedoom (1917) found that among the rats
of India and Java respectively there occurred highly localized
groups with distinctive characters, often consisting of a few
individuals only. Sometimes the distinctions seemed to be mono-
factorial, sometimes to depend on several different genes; in
some cases the groups disappeared after quite a short time. In
this case we have to do apparently with the effect of chance
inbreeding on one or a few recessive genes; it is of interest,
however, in demonstrating the high potential of variation avaE-

THE SPECIES PROBLEM; GEOGRAPHICAI. SPEQATION 203

able, through which geographical difFerentiation may appear in
the event of complete isolation or of partial isolation accompanied
by cMerential selection.

Gilmour and Gregor (1939) have recently proposed the term
deme for “any specified assemblage of taxonomicaUy closely
related individuals”. This should be useful to replace such
cumbersome phrases as “local intrabreeding populations”. The
ultknate natural unit in sexually reproducing species is then the
deme, and analysis is needed to show to what extent demes are
isolated from each other (see also Buzzati-Traverso et al., 1938).

In some instances, new types have been thrown up which
spread from their centre of origin owing to some selective
advantage, thus causing local differentiation of a peculiar type.
When this occurs in an isolated population, the new type may
oust the old within the area. This has happened with the melanic
form of certain species of the bird Coereba in the West Indies
(p. 94 n; Lowe, 1912), and is in progress with the melanic type of
the opossum Trichosurus vulpecula in Tasmania (p. 104}. It may,
however, also occur in large or continental populations, as with
the simplex tooth-character of Microtus arvalis in N. Germany
(p. 105). Whether the resultant gradient in proportions o£ simplex
and normal teeth will reach an equilibrium, or the simplex
character will infect the whole species, remains for future genera-
tions of taxonomists to determine (p. 105).

In general, we may be sure that the analysis of invisible physio-
logical characters, and the more intensive study of visible ones,
will reveal that species are much more diversified geographically
than is now generally recognized (for further examples, see
Timofeeff-Ressovsky, 1940).

"Wherever there is any appreciable isolation, not only will
non-adaptive distinctions accumulate, but adaptation to local
conditions will be able to proceed to a further pitch than where
counteracted by free gene-flow. Further, internal (intra-group)
clines (p. 220) will doubtless be revealed within populations of
species which are not too mobile.

Out of these minor local differences, die processes of differ-
entiation win create the obviously distinct groups which we call

204 evolution; the mooekn synthesis ^ ^

subspecies and species, and tbe obvious regularities of inter-group
rlinps But tbose which merit taxonomic naming will form but
a small fraction of the total amount of geographical diversification.

Mention should here be made of the views of WilMs (1922,
1940). Chiefly on the basis of studies of geographical distribution,
he entirely rejects the selectionist view, and concludes that evolu-
tion is a largely automatic affair of differentiation produced by
large mutations, followed by spread of the new type at a more
or less constant rate, and by further difierentiative variations in
due course.

Unfortunately most of Wilhs’ conclusions are vitiated by his
failure to take account of modem work. Thus he continues to
accept Fleeming Jenkin’s criticism of Darwin, namely, that new
variations will be swamped by crossing, whereas, as R. A. Fisher
in particular has shown (see p. 55), this objection has been
entirely obviated by the discovery that inheritance is particulate.
He adopts, in exaggerated form, de Vries’ idea of large mutations,
and appears to be unaware of the modem conception of the
adjustment of mutations to the needs of the organism (p. 67).
He does not refer to polyploidy as an evolutionary agency in
higher plants. He makes a sharp distinction, which is quite un-
justified on general biological grounds, between structural and
functional adaptations. He concludes that, since localized endemic
forms, e.g. on islands or mountain-tops, appear to have no
adaptive value, they must have arisen by sudden mutation,
whereas “drift” due to accidental recombinations in small popu-
lations wdl clearly account for a great many of such cases (p. 58).
It seems, further, thit he has not adopted the principle of geo-
graphical replacement as a basis for taxonomy. If this were done,
many of his endemics would doubtless turn out to be, not new
full species, but new subspecies produced by “drift”, and it
would be much easier to distinguish between such products of
recent diversification and tme relicts. He practically ignores
zoological facts, notably in paleontology, which contradict some
of his general conclusions such as that gradual adaptive improve-
ment does not occur, that no important change is to be found
in m^or groups during geological time, and that the distinctive

THE SPECIES PROBtEM: GEOGRAPHICAl. SPECIATION 20 $

characters of moderately large taxonomic units are not adaptive.
He does not distinguish between euryplastic and stenoplastic
fornas (p. 519), or between those which are narrowly adapted
and those which succeed by virtue of general vigour and viability.
In conclusion, he neglects all the evidence that new types may
arise in several quite distinct ways, and maintains that there is
only one mode of evolutionary difierentiation.

If the extensive data which he has assembled could be analysed
in the light of modem knowledge, instead of being lumped
together to produce a heterogeneous mass from which purely
statistical consequences can be drawn, it is probable that certain
valuable conclusions could be reached. It is likely, for instance,
that his general idea of “age and area”, or progressive increase
of range with time, would prove to hold for a number of forms,
and to have interesting consequences. The further conclusion,
arrived at in conjunction with Yule (Yule and Willis, 1922)
that differentiation is also a function of time, and that genera
tend to split into two at more or less regular intervals, may also
be of importance, though, as examples such as Lingula or Nautilus
demonstrate, it is certainly not universally valid.

He has also collected a number of very interesting facts con-
cerning the number of species in different genera of a family.
The average number of species per genus in flowering plants
(apparently without taking into account the principle of geo-
graphical replacement) is 14 or 15. But there are in all families
a very large proportion of unispedfic genera. Thus more than
a third of the genera of Compositae (446 out of 1143) atid of
those of Garyophyllaceae (39 out of 78) are monotypes, with
only one species each, indicating a very peculiar form of differen-
tiation. Further, the largest genus of a family is always relatively
enormous in the number of species it contains, in over 40 per
cent of cases (235 famflies) comprising half or more than half
the total number of species in ^ family. Facts such as these
demand the most careful consideration. However, we can be
sure that their meaning will not be elucidated by the purely
statistical methods used by Willis, but must wait upon the fullest
analysis, notably ecological and cytological.

'306 evolution: the modern synthesis

4.. CLINES {character-gradients)

The delimitation of named subspecies in different areas, each
with their own distinctive mean and range of variability in
respect of a number of characters, provides one means of pigeon-
holing the data of geographical diflerentiation. But, as we have
already seen in discussing such cases as that of Peromyscus, this
method does not cover a certain 'aspect of the facts, namely the
frequent tendency of characters to change gradually and continu-
ously over large are^

In point of fact these character-gradients, or dines, to give
tliem a convenient technical name (Huxley, I939<j, I939^)> appear
to be much commoner than is generally supposed. Indeed, on
any general Darwinian view, we should expect to find them as
one of the general features of organic variation. Natural selection
will all the time be moulding life adaptively into its environment;
and since gradients in environmental factors are a widespread
feature of the environmental mould, we should expect organisms
to show corresponding adaptive gradients in their characters.

The adaptive characters directly affected may be visible
characters such as absolute size, or relative ear-size in warm-
blooded animals; or they may be invisible, physiological features
with no outward sign in the characters usually employed in
taxonomy (e.g., the difierence in temperature-resistance in
different regional populations of Drosophila (p. 191) ; or the
temperature-preferences of the races of the beetle Carahus nemoralis
(Krumbiegel, 1932) ; or, as with the phototropism of the same
species, they may be associated with slight differences in eyc-
structure; or finally, and it appears most frequently, they may be
physiological features reflected in non-adaptive but taxonomically
convenient correlates such as proportion of parts, or colour.

Broad environmental gradients exist in numerous general
climatic factors, in relation primarily to latitude and altitude.
Such graded climatic factors include temperature, humidity,
solar intensity, relative day-length, and so forth. More restricted
gradients are found in ecologicd factors, in relation to the change
from one habitat to another — gradients in salinity or water-

THE SPECIES problem: GEOGRAPHICAL SPECIATION 2O7

content, in height of vegetation, in edaphic conditions, and so
on. There is, of course, no sharp line to be dravui between
geographical and ecological gradients. The gradient up a steep
mountainside may be better styled ecological; but in many
features it will repeat the general geographical gradient from the
base of the mountain to higher latitudes. The point is that such
gradients exist, and that they are of every size, from those of
largest scale between the equatorial and polar regions, to those
of extremely small scale like that in decreasing moisture round
a pond.

How may wc expect life to accommodate itself to these graded
features of its environment? In the first place, their scale has an
influence. Because of the rate of gene-flow through a population,
a cline cannot usually establish itself as a characteristic of an inter-
breeding group unless the group covers a considerable area. The
only way by which clincs on a genetic basis may be established
over small distances is by having a highly variable population of
which diffi;rent types are adapted to diflerent ecological conditions.
Selection will then automatically see to it that different propor-
tions of the various types are found along the environmental
gradient, even when this is quite short. Short clincs of this type
do exist, as we shall see later, in certain plants, c.g., Plantago
maritima {•p. 223). They are not, however, enduring characters
of the species, but come and go within its plastic framework
with the changes in ecological conditions. They will also tend to
be repeated, con variazioni, 'm many localities, wlhlc large-scale
climatic dines will be few in number, and will constitute charac-
ters of the species as definite and enduring as its measurements
or its geographical range.

With regard to large-scale clincs, the biological peculiarities
of the species will of course have an influence, large size and high
mobility tending to make them less prominent, and vice versa
(sec p. 239).

Any continuously-graded variation will tend fo be broken up
by various factors. In the first place the accidents (biologically
speaking) of complete or almost complete geographical isolation
will introduce discontinuities. These will interrupt gene-flow.

20H l•VO!-UrlON: TOli MOIHtRN SVNT

and so not only allow local selection to act more cftcctivcly (as
we saw with the island subspecies of palmnotus,

p. i86), but also permit the Scwil Wright effect of noii-adaptivc
differentiation to occur wherever the isolated populations arc
small* The first effect will tend to break up a continuously sloping
dine into sharp steps, wliilc the second will impose non-adaptivc
features upon it, sometimes quite obscuring any uiidcrlyijig
regularity.

Biological discontinuities will also break up the continuity of
dines. Here again, nco-mcndclian principles have pointed the
way to important deductions. We have shown in an earlier
chapter how the effects of major genes arc selectively adjusted,
individually and mutually, by means of combinations of modi-
fiers to suit the needs of the organism, notably in giving maximum
vigour and fertility. There is an internal adaptation of the gene-
complex as wtU as an external adaptation of characters. This
extension of the principle of genie balance we may call the
principle of harmoniously-stabilized gcnc-complcxcs.

Let us now consider what will happen witliin a continuous
population spread over a large area in which markedly diiicrcnt
climatic conditions occur in different regions, but with the
extremes connected by environmental gradations. Selection will
then be operative and will tend to adapt the population locally;
however, this local adaptation will be impeded and graded by
gene-flow. But wherever some accident, such as temporary or
partial isolation, allows selection full scope, local adaptation will
be intensified, and the major adaptive genes will be fortified by
internal adaptation until a local harmoniously-stabilized gene-
complcx is built up. Once this occurs, the resultant extra vigour
and fertility will permit the bearers of this gcnc-complcx
to spread beyond the area to which they were originally
adapted.

If several such gcnc-complcxcs arise within the area of the
species, they will tend to spread until they meet. As Sumner
(1933, p. 76) has stressed, local groups must be regarded as in a
dynamic equilibrium based on relative population-pressures. He
compares them, to a scries of balloons in contact, the population-

THE SPECIES PROBLEM : GEOCJRAPinCAl SPECIATION 309

pressures being here represented by the gas-pressures in the
balloons. Groups with high population-pressure, resulting from
successfully stabilized gene-complexes, will spread, and groups
whose relative population-pressure is below a certain threshold
may actually be extinguished, their remnants being incorporated
into and transformed by the more successful groups (see p. 187).

What his simile does not explain, however, is the permanence
of the skin of the balloons — as represented in nature by the
relatively sharp delimitation of subspecific groups. As we have
seen (pp. 183 seq.), in many cases, adjacent subspecies are separated
by a relatively narrow zone of intergradation. What maintains
this zone i Why does not gene-llow broaden it and break down
the sharp distinction between the two subspecies ?

On the principle we have been following out, the answer is
simple. Crosses between two harmoniously-stabdized gcnc-
complexes will give relatively disharmonious gc le-combinations.
The zone of intergradation will constantly be renewed by inter-
crossing; but it will as constandy be prevented from spreading
by selective elimination in favour of the better internal adaptations
on either side, even though it may shift its position (p. 249).

This principle doubtless also explains why the zone of
recombination between two markedly distinct yet interfertilc
forms which have met after differentiating in isolation, in some
cases remains so narrow, notably in the crows (p. 248).

We here meet with a new type of biological discontinuity — a
partial discontinuity, as opposed to the complete discontinuity
found between full species. Where the “biological tension”
between different portions of a widespread species is sufficient,
a condition of equilibrium will be reached, represented by a
series 5f distinct subspecies passing into each other by inter-
breeding at narrow zones of intergradation.*

This will be facilitated by partial environmental discontinuities
such as partial barriers, or unfavourable zones where population-

^ A suggestive ecologicai parallel exists in the way in which relatively uniform
biotic coininuin'rics pass into each other across narrow intermediate zones (sec
Elton, 1927, i'h. i). Such zones arc sonicdincs styled “tension zones” (cf. Elton,
1938). hi both cases, environmental continuity is reflected in partial organic
<liscoiithiuity.

210 evolution: the modern synthesis

density is lowered; it will also be facilitated by sharp changes in
environmental conditions, as where a mountain range rises
abruptly from a plain, or open country gives place suddenly to
forest. But— and this is important — ^it may occur in the absence
of any barriers or any abrupt alteration in the environmental
gradient: the cause of partial discontinuity is then a purely bio-
logical one, due to the nature of the gene-complex.

So far, these deductions, however their vaHdity be supported
by the frequent existence of subspecies separated by narrow zones
of intergradation, have only been experimentally verified in one
instance. TimofeefF-Ressovsky (i 932 l>), studying the geographical
varieties of certain lady-beedes, found that their visible peculiari-
ties depended on several mendelian genes, and that the combina-
tions of these actually realized in wide-ranging geographical
groups were almost invariably more viable and more resistant
than the recombinations not found in nature, which he produced
by crossing. It is much to be hoped that further experimental
analysis of this important point wiU be made in other types.
Meanwhile Sumner’s data in Peromyscus poUonotus show that the
population of the narrow intergraing zone between P. p. poUo-
notus and P. p. albifrons shows a markedly higher coefficient of
variation than either pure subspecies (see p. i86, and Huxley
I939<i), a fact which is to be expected on the above theory of
harmoniously stabilized gene-complexes.

If, as it seems probable, these deductions prove valid, it will
mean that subspecies, as found in nature, are in reality of two
distinct types. The first we may call independent, and consists
of those which are so fully isolated that gene-flow between them
and other groups is wholly or virtually interrupted; the second,
or dependent, are those we have just been discussing, which
interbreed with thek neighbours along uitergrading zones.
Independent subspecies may differentiate into full species, and,
with sufficient time, normally will do so. Dependent subspecies
normally will not do so, but though they may continue to
evolve, will evolve as part of the whole interbreeding complex
to which they belong. Thus it is not true to say that subspecies
are necessarily “species in the making” (as was done, for

T! IE SPECIES PROBLEM : GEOGRAPHICAL SPECIATION 2 1 1

instance, by Rothschild and Jordan, 1903) ; some arc, and some
arc not.

The breaking up of a continuous population into subspecies
by the physical discontinuities of geograpliical barriers and the
biological partial discontinuities of narrow intergrading zones
will profoundly modify any cline systems present. The continu-
ously sloping character-gradient that simple a priori considcratioijs
might lead us to expect is converted into a staircase or a stepped
ramp, the separate subspecies corresponding with the treads, flat
or gently sloping, and these being either united by steep slopes
— ^the zones of intergradation— or, in die case of completely
isolated subspecies, remaining unconnected. The mean or modal
values for the several subspecies will often fall on a gradient.
This may be called an external or intergroup cline; when the
characters of a subspecies change slighdy or gradually across
the area of its distribution, giving a sloping tread in the staircase,
we may speak of its showing an internal cline.

Intergroup dines are a very frequent feature of geographical
differentiation, and appear usually to be correlated with corre-
sponding gradients in environmental features, though Mayr
(1940) dtes some dines in tropical birds where no such corre-
lation can be found. A summary of die chief generalizations
concerning them is to be found in Rcnsch {193 3 a, 19380), and
an excellent discussion in Goldschmidt (1940). The most iinpor-
tantof these have been called Bergmann’s Rule, AUen’s Rule, and
Gloger’s Rule, after their most important proponents. They lead
to much parallel variation in related spcdcs, though all of them
are broad correlations only; widi a considerable number of
exceptions.

Bergmann’s rule may be stated thus. Within a polytypic
warm-blooded species, the body-size of a subspecies ususJly
increases with decreasing mean temperature of its habitat. A
detailed statistical study by Rcnsch showed diat in the great
majority of cases this rule holds good for birds. For Corvidae
and Piddae there arc hardly any exceptions, and in general the
rule applies in 70 to 90 per cent of cases. The rule also applies to
mammals, though here the exceptions are more numerous, it is

212 evolution: the modern synthesis

dear that size may be modified in other ways, e.g. by selection
in relation to type and abundance of food. We have already
noted the fact that the size-gradient between forest and open-
country forms runs in opposite directions in buffaloes and squirrels
in Africa. The reason for the greater number of exceptions in
mammals is doubtless to be found in various biological peculi-
arities of the group, such as hibernation, temperature-regulation
by means of greater or less growth of hair, nocturnal habit, use
of burrows and dens, etc.; thus 5he burrowing Microftis behaves
contrary to Bergmann’s Rule (Dale, 1940). Rcnsdi has shown
(i939f>) that the correlation in temperate regions is with winter
minimum temperature. This is what would be expected, selection
being exerted by the most rigorous conditions. It may be
prophesied that in semi-tropical areas the correlation will be with
the maximum temperature in the hot season.

Recent studies (Salomonsen 1933, Huxley I939fl) have enabled
a beginning to be made with a quantitative study of Bergmann’s
rule. Thus for three polytypic spedes of birds in western Europe,
a change of i per cent in wing-length requires a difference of
2® N. latitude in the redpolls (Cardwe/is flammed), of just over i®
in the puffins {Fratercula araica) , and of only a little over 0*5° in
the wrens {Troglodytes troglodytes). Oiiiet measures of size (beak
in puffins, tarsus in wrens) show approximately the same rate of
change as the wing, indicating that the effect is on the animal as
a whole. The total relative change is least in the wrens (about
12 per cent of lowest wing-length), and highest in the puffins
(nearly 50 per cent), but the range of the last-named is from
Majorca to Spitsbergen, whereas the size-cline in the wrens is
only exhibited between the N. of Scotland and Iceland, the
wren population of mainland Britain and western Europe being
very stable. Such chfferences presumably result from differences
in selective intensity, but it is difficult at the moment to see why,
e.g., there should be less effect in the tiny redpolls than in the
relatively large puffins.

In cold-blooded animals, matters are more complex, types
often appearing to have an environmental optimum where they
attain tlicir maximum size. In frog spedes, forms from colder

THi; SPliCIIiS I’ROBLr.M : GF.acUAPinCAL StM-CriATION 313

climates seem to be larger, witir relatively shorter hiiid legs
(Porter, 1941; and see Pfliiger and Smith, 1883).

AUen’s rule also is correlated with temperature. It states that
in warm-blooded species, the relative size of exposed portions
of the body (limbs, tail, and ears) decreases widi decrease of mean
temperature. We have already noted diis for Percmysctis species.
Statistical treatment showed that it appUed in 80 to 85 per cent
of small mammals investigated by Rensch, and to almost the
same extent for wing-length in birds (five families of non-
migratory North American birds).

This rule also appears to hold for related species as well as
related subspecies; c.g. for ears in foxes (Hesse, 1924). When the
temperature is at all extreme, ear-size is of considerable adaptive
value, small ear-size reducing heat-loss in cold climates, large
car-size facilitating heat-loss in hot climates.

Rensch (19384) has shown that Allen’s rule is in part purely a
consequential effect of the negative allomctry of the parts con-
cerned, but that this must in many cases have been accentuated
by selection in relation to heat-loss (see p. 547).

Both Bcrgmaim’s and Allen’s rule may be included under the
more general principle that in homothermous forms body-
surface relative to bulk tends to decrease with decreasing outer
temperature.

These effects prove to be genetic in every case as yet tested.
It is noteworthy, however, that temperature also has a direct
effect of the same type on such organs, but the modification is
not permanently inherited (Przibram, 1925).

Pigmentation also shows marked geographical gradients, but
these are rather more complex. Glogcr’s rule applies to pigmen-
tation in warm-blooded species. In its modem formulation it
states that intensity of melanin pigmentation tends to decrease
with mean temperature (though the operative factor may possibly
be Hght rather than temperature) ; however, humidity also has an
effect, great humidity together widi high temperature promoting
the formation of the black cumelanins, wliilc aridity togedicr
with liigh temperature promotes the substitution of the ycllowish-
or reddish-brown phaeomelanins. Phaeomclanins tend not to be

THE MODERN SYNTHESIS

found in cooler conditions even if arid. Thus the maximum depth
of pigmentation will be found in humid and hot climates, the
minimum in arctic cHmates. Heat and aridity, as in subtropical
deserts, will promote yellowish and reddish browns, while lower
temperature and aridity, as in steppes, will promote greys and
grey-browns.

Among numerous examples, the studies of A. Roberts (1935,
1938) on S. African birds may be cited, though he is inclined to
find geographical regularities also in other colours and in striping.
Rcnsch’s statistical investigations showed Gloger’s rule to apply
in 85-90 per cent of cases. We have seen a good example of the
results in African squirrels (p. 192). The African buffaloes (C.
Christy, 1929), with their red forms in forests and black forms
in open country, constimte an exception. Lipochrome pigmenta-
tion tends to be of lower intensity in hot arid regions.

Invertebrates also show pigmentation-gradients, e.g. bumble-
bees, wasps, beetles, butterflies; but these are complex (see p. 262).
Lizards (Gerrhonotus) in western North America show distinct
clines (Fitch, 1938), size and relative tad-length decreasing with
decrease of temperature. Dobzhansky (1933), by genetic analysis
in lady-beetles, has made it possible to demonstrate a genetic
chne underlying geographical variation in Hamonia axyridis.

In aU such cases, since related forms will tend to show similar
effects, parallel evolution often results. Vogt (1909 and 1911)
gives numerous cases among bmnblebees {Bombus), and G. L.
Bates (1931) among West African birds. Aldrich and Nutt
(1939) find that in Newfoundland all resident birds which exhibit
any geographical variation are exceptionally dark, often more so
on the more humid eastern coast. An excellent example is given
by Mayr and Serventy (1938) from birds of the Australian genus
Acanthiza. Several species show a concentric arrangement of
subspecies, those in the arid interior being pale, while those on
the ffW. coast and a small part of the less humid S.E. coast are
very dark. An interesting feature of this case is that the boundaries
of the subspecies do not always overlap exaedy in different
species, but may run parallel at some distance from each other.
Mayr and Serventy are inclined to interpret this on the basis of

THE ' SPECIES ■ PtOBLEM : ' CEOGR APIHCAI ; SPECIATION 2 1 5

difFcring rates of evolutionary adjustment to ciivirGiiinent. It is,
however, just ' as likely that the pigmentary expression of what- '
ever physiological adaptation is involved, may ■ differ from
species to species. See also Rcnsch (1936).

Another interesting case coiiccms the crested larks {Gakrida)
of N. Africa and S. Europe (Rothschild and Hartert, 1911). Two
closely allied species, G. cristata and G, theklac, largely overlap
in range, but are ecologically differentiated. Both have numerous
subspecies, which show parallci variation in coloration correlated
with soil-colour (though complicated by polymorphism in G*
theklae). G, theklae also shows a dine in song, which becomes
more prolonged as one passes from north to south in Africa.

Numerous other geograpliical dines appear to exist. The
number of eggs in a dutch increases with increasing latitude
within bird species, and the form of the wing becomes more
pointed (Rcnsch, 1938?)); organisms tend to decrease in size with
decrease in salinity (e.g. in the Baltic; but this may be only a
non-genctic modification) ; the number of fm-rays and vertebrae
in many fish varies inversely with temperature; relative heart-
weight decreases with temperature ill wrarm-bloodcd species;
tropical conditions promote a reduction of stomach- and intestine-
size in species of birds with a mixed diet, etc.

The selective interpretation of such dines gives a rational
basis to the Geograpliical Rules of Bcrgmann, Allen, etc., which
we have just discussed; and to the consequent parallel variation.
This is well discussed by Goldschmidt (1940), p. 83, who sub-
sumes all the Rules under the head of “parallelism of subspecific
dines'’. Tliis parallelism may lead to forms which arc taxono-
mically indistinguishable being evolved independently in several
areas. Under current taxonomic practice, these arc lumped
together under one subspecific name. Thus the woodpecker
type named Picus cams sanguiniceps appears to have evolved
in the Western Himalayas, Southern Malacca, and Cochin China
(Danis, 1937); and Mayr and Greenway (193^) state that in the
bird Mesia argentauris three populations which “differ, though
too subtly for formal description”, although probably not geneti-
cally related, will all liave to be called M. a. argentauris, Tliis is a

2i6 evolution: the modern synthesis

clear case for subsidiary taxonomic terminology (p. 405 ). whether
specified in the form of clines or descriptive ecological terms.

In addition to such general or widespread gradients, mani-
fested in many related and unrelated species, others appear to
exist which apply only to limited groups (Rensch, I933<i)- From
among the wealth of possible examples we may adduce a few
more concrete instances to illustrate the principle.

We have mentioned the gcograpliical variation of the gipsy-
moth, Lytnantria dtspar. Goldschmidt (i934> P- ^7o) sumrnarizes
the geographically-varying characters which he has investigated
genetically. These include (i) characters wliich, in his view, are
definitely adaptive: — the male and female sex-factors, which
differ in potency; the length of larval development; the length
of the diapause; (ii) characters which Goldschmidt considers
undoubtedly to be correlated with other distinctions which are
adaptive: — ^the number of moults (four in both sexes; four in the
male and five in the female; five in both sexes); the total size
(weight) of the animal; the larval pigmentation; the colour of
the imaginal abdominal hair; and (iii) characters which seem to
have neither direct nor indirect (correlated) adaptive value:—
the imaginal wing-colour.

Clines appear to exist in regard to many of these characters:
in general these arc very gentle in the main holarctic land-mass
but much steeper, and with more tendency to sharp breaks, in
tlie eastern Asiatic region. However, the clines for different
phenotypic characters are not always coincident.

This species is the only one in which a full mendclian analysis
has been made of the genetic basis for geographically-varying
characters. It is interesting to find that most are controlled by a
series of multiple alleles, whose effect is often reinforced by
cytoplasmic influence. Length of diapause, however, and colour
of abdominal hair are determined by a set of independent multiple
(polymeric) genes; and wing-colour appears to be determined
p£utly by a series of multiple alleles, partly by four other inde-
pendent genes. (In Perontyscus, Sumner foimd tliat almost all
subspecific differences depended on several genes, which he
considered to be independent multiple factors.)

THE SPECIES problem: GEOGRAPHICAL SPEOATION 317

A peculiar dine is found in the insular populations of the deer-
mouse, Perotnyscus mankulatus, on the islands of Georgia Strait,
British Columbia (Hall, 1938), in relation to distance from the
mainland. Within a mere fourteen miles, body-length increases
from 84 to T03 mm., and tail-length decreases from 94 to 66 mm.,
almost halving the tad-body ratio. It seems impossible to correlate
this with any of the usual geographical rules.

The small copper butterfly, Heodes phloeas, analysed by Ford
(1924) , shows distinct gradients in certain regions, while in others
the distribution is irregular, and in stiU others, such as North
America, there is hardly any variation over large areas. Ford
considers that this last fact is due to the species having only
recently colonized the region, so that there has been inadequate
time for geographical differentiation. Certain characters of the
swallowtail butterfly, Papilio dardams, show a graded distribution
(Ford, 1936), as do some of Acraea johnstoni (Carpenter, 1932).
hi this and other butterflies with polymorphic females, a poly-
morph-ratio gradient in the proportions of the forms may often
be observed (Eltringham, 1910; Carpenter, 1932), as is also the
case in the polymorphic foxes (p. 103), the guillemot (p. 105),
etc. The fulmar petrel, Fulmarus glacialis, shows a condition
intermediate between the dimorph-ratio cline (in the propor-
tions of two sharply distinct forms), and the continuous gradation
(J. Fisher, 1939!^. Here there is a primary distinction between
pale and dark (blue) forms, but the blue types exist in various
degrees of intensity, and there is a chne towards a greater pro-
portion of the deeper blue types in the far north.

The fox-sparrows studied by Swarth (1920) show character-
gradients, but these are by no means simple. The sharp steepening
of the gradients at the zones of intergradation between subspecies
is again prominent. In some regions, different trends occur in
different directions. Finally, there are certain apparent anomalies
in the trends. Swarth suggests that these depend upon migratory
habit, since the type would be influenced (whether by selection
or otherwise) by conditions in their winter range as well as by
those in their breeding quarters.

The zebras of the BurcheU’s zebra group {Equus burchclli) show

218 evolution: the modern synthesis

an interesting dine (see Shortridge, I 934 )- The equatorial fornis,
covering two-thirds of its north-south range, are fully striped,
south of the Zambesi the striping is progressively reduced, first
on the tail and legs (E. b. hurcheUi), and then on the hinder half
of the body (£. h. quagga, the true quagga, now extinct, and often
regarded as a separate species). Here a direshold value for striping
has been reached at a certain latitude.

An interesting case is that of the cole tit, Pams ater. One of the
characters by wliich the Irish subspecies, P. a hibernicus, is distin-
guished from the British, P. a. britamicus, is the amount of
yellow lipoclirome pigment in the plumage, manifested, especial y
in the yellowish colour of its under-parts. Occasional spedmens,
however, lack this feature and these are more common towards
the north-east of Ireland; and occasional spedmens from Wales
show varying degrees of the characters of the Irish form
(Witherby, 1938-41). It would seem that the Irish Chamicl has
introduced a considerable discontinuity into a coloration-gradient
(see Huxley, I 939 <j)-

In this case, it is interesting to note, the various forms appear
to differ primarily in regard to rate-genes (p.528 ft) aftectingthe
rate of deposition and final amount of Hpochrome pigment.
This seems jJso to be the case in the African buffaloes just men-
tioned, though here the pigment concerned is melanin (see
discussion in Ifuxley, i 939 <i)- A similar case is that of the Rassenkreis
of the palearctic go^v/k Accipiter gentilis (Gladkov, 1941 )- The
subspecies to the N. and E. are hghter, and in them the young
birds, which are always darker than the adults, show an carher
onset of the lightening process. This spedes also obeys Bergmann s
rule.

Ih the case of Pams afcr we have apparendy an approximation
to the condition of the stepped ramp, in which the subspedcs
show internal chnes. A similar example has already been men-
tioned in Peromysr«s po/io«otHs (p. 187). In both these cases the
internal clines of certain subspecies appear to be confined to the
Tnargins of the areas of distribution, while in P. poUonotus z
pigmentary dine is continuous across the whole subspecific area.

Among the silver pheasants (Beebe, 1921), Gennaeus shows

run siHicifs ci:0(;raphk:al sm:jATi(>N 219

large-scale colour-clines in all the main species. In addition, there
has been considerable hybridization along the bDiindarics of
species or marked subspecies, producing irregular gcnocliiics.
The iiitcnial dine in the moth Platysamia (Sweadiier, 1937)
appears to be a geiiocUne, due to hybridization between two
distinct forms brought together by post-glacial migration. The
dines in die frequency of blood-group genes in man across the
Palearctic (Haldane, 1940) arc also undoubtedly due to migra-
tion; where natural barriers occur, the slope of the dines is much
steepened.

L0ppcnthin (1932) describes a continuous dine in tlifc colour
of the under parts (from chestnut to pure white) in the coniinon
nuthatch {Sitta caesta) from several hundred miles from west to
east across north-central Europe, At cither end, the dine passes
over into forms which are stable over considerable areas — i.e.
gCGgrapliical subspecies. It is possible that here, too, we arc
dealing with a genodine resulting from hybridization between
two distinct forms which have met subsequently to dijfFercntiation;
possibly, however, it is a true geographical internal dine related
to an environmental gradient, and such dines may turn out to
be commoner than now supposed. The differentiation of the
squirrel C^llosdums sladeui along the Ghindwin river is considered
by Thomas and Wroughton (1916) to be into numerous sub-
species: their own data, however, make it probable that it is
really into two colour-clincs, separated by a tributary (see p. 227).
Again, Fleming and Snyder (1939) in the song-sparrow Mclospiza
melodia £md a continuous N W,~S,E. colour-cline across Ontario.

L. L, Snyder (1935), in a study of the sharp-tailed grouse
{Pcdioecetes phasianellus) of North America, finds that, in addition
to distinct subspecies of the usual type, there is a colour-clinc
from north to south over the Great Plains. He hesitates whether
to give trinomials to various forms witliin this dine. Although,
as he says (p. 59), this would be quite a normal procedure accord-
ing to present taxonomic practice, this does not mean that it
would be justified (see p. 226),

We may be sure that many forms have been accorded sub-
specific rank because the conferring *of a trinomial was the only

220

evolution: thb modern synthesis

accepted method of distingmshing them, whereas in reahty they^
represent only points on a continuous cline. Once the idea ot
dines is generally accepted, we may safely prophesy that an
increasing number of cases of dines will come to hght, often

replacing series of subspecies. .

Examples of continuous (internal) geodines withm extensive
populations are as yet infrequent, doubdess because they are tes
readily detected. However, a very interesting case is tlmt ot the
honey-bees, studied by Alpatov (1929)- hi the plam of European
Russia, a gradient occurs in tongue-length. This mcrea^s from
north to south so regularly that the change can be reasonab y we
represented by a madiematical equation connecting tongue-
length and latitude [y = 10-3219 - 0.07559^. wlf re j = tongue
length in mm., and x = degrees of N. latitude). The tongue-

length ranges from 5*726 mm. to 6-733 mm. 1 1 •

A north-south gradient towards smaller absolute body-size,
larger relative leg-size, rdatively broader wings, tighter abdomen-
colour and other characters, including certain points of behaviour,

is also apparent. ,.11

In the Caucasus, this gradient is continued with decreasmg

latitude for tongue-length and relative leg-length, but is reversed
(presumably in relation to decrease of temperature with altimde)
for abdomen-colour and relative size of wax-glands. This shows
how valuable the specification of character-gradients may be as
an additional method of taxonomic description, as does fact
previously cited (p. 217) that in fox-sparrows (PtJsserel/fl) different
character-gradients run in different directions; the same is true
(oiLymantria (Goldschmidt, 1940, pp. 69-70, 84).

No such graded variation is to be found in North America:
this is due to the fact that the honey-bee is there a recent importa-
tion. The geographically-^aded characters are all or mostly
genetic. Some of them appear to be adaptive, e.g. the tongue-
length in relation both to the type of flora and the average level
of nectar in the flowers.

Another interesting case is that of the lady-bird beetles {Coccin-
ellidae) studied by Dobdiansky (193 3 )* ht many of these, poly-
morphism eidsts, several qualitatively distinct non-intergrading

THE SPECIES problem: GEOGRAPHICAL SPECIATION 221

types being foimd within the species. Character-gradients of two
sorts are foimd in the group, one concerning the frequency of the
different qualitative types, the other affecting the quantitative
development of a single character of a particular type or types.
Hiunidity, and to a certain degree low temperature, appear to
favour depth of pignientation, though the correlation is by no
means complete. Various species show well-marked pigmentation
dines around centres of light and of dark forms.

This example is interesting as combining the two types of
internal or intra-group dines — ^in quantitative characters and in
polymorph ratio (p. 103).

A similar combination seems to exist in the gyrfalcon {Falco
rusticolus). Its various forms may prove to be better represented
by a single chne (involving an increase of size and in percentage
of hght-coloured birds with increasing N. latitude) than by the
usual method of subspecific namii^. Witherby (1938-41) dis-
tinguishes a very dark subspedes from Labrador, a moderately
dark (typical) subspedes from the north of the western palearctic,
a moderately fight subspedes from Iceland, and a very pale
subspecies from N. Greenland, as well as others from Siberia
and Arctic Canada.

However, in some localities a certain number of contrasted
types occur. Thus a minority of N. Greenland birds are indis-
tinguishable from the typical Iceland form. In Iceland, there is
a considerable range of variation, and a typical Iceland form and
one similar to the pale Greenland type have been recorded in the
same brood. The S. Greenland population is indistinguishable
from that of Iceland. The “subspedes” from Siberia and Altai
show great variation, and can only be separated on die basis of
the relative abundance of the various types they contain.

Bird and Bird (1941) state that the very dark forms arc in the
great majority in Labrador, but that some occur in S.W. Green-
land. Variation is at its lowest in N.E. Greenland, and they wish
to restrict the N. Greenland subspecies to this area, while admitting
that some birds from Arctic Canada arc as pale. They lump all
the birds from Labrador, Arctic Caiiada, and S. Greenland into
one subspecies, in -spite of the great local variation, and in spite

222

EVOtUTION: THE MODERN SYNTHESIS

of the identity in colour of the S. Greenland widh the Iceland

birds. ■ ^

It would appear much more logical to include the whole
population in a single cline; the relative lack of variation in the
N.E. Greenland birds would then be due to their being close to
the limit for pale colour.

Juveniles are always darker than adults, but those forms with
lighter adults have lighter juveniles. The variation is therefore quite
possibly genetically dependent, like those in buffaloes and cole tits
(p, 218), on rate-genes affecting the rate of deposition of pigment.

Polymorph-(dimorph-)ratio clineshave a special interest for the
selectionist, since the continued existence of two or more sharply
marked types within a population implies a selective balance
between them (p. 97 > Ford, 1940^)* When a cline exists

in the proportion between the two, the geograpliical conditions
along it may give a clue to the selective factors involved.

A recent study of primroses (Primuh pulgaris) by Crosby (1940)
shows how the origin of a mutation with positive selective value
may give rise to a temporary polymorph-ratio cline. Primulas
normally show heterostyly with the two types, pin and thrum,
approximately equal in frequency. In one area, however, large
numbers of long homostyles were found. If, as seems probable,
these are normally self-fertilized, their numbers will increase, and
those of the other types decrease, thrum more so than pin. If
the mutation arose in one centre, it would spread, and the ratios
of the three types would change with distance from the centre
and with time. The preliminary counts so far made are not
conclusive, but do not contradict this hypothesis (see p. 313).

A remarkable cline in regard to sexual dimorphism, but
affecting species instead of subspecies, is found in the flycatchers
{Pomaea) in the Marquesas. The northernmost, species,
has pied black-and-white males and brown females; the central
P. mendozae h^is black males and probably pied females; and the
southernmost P- udiitneyi is black in both sexes (Murphy, 1938),
Colman (1932) has made careful measurements on the shells
of the periwinkle, Littorina okusata. He finds great variability in
size and form, but the populations from the two sides of the

THE SPECIES PROBLEM : GEOGRAPHICAL SPEaATION 223

Adanfic cannot be distinguislied statistically. Here and there,
distinct gradients occur. For instance, in passing up the New
England coast a marked change in shape occurs along a portion
of Maine, the shells becoming thinner, with taller spires. Biometric
investigations of this sort on molluscs with a wide range should
provide useful data linking ecology with systematics.

The apphcation of the principle of geograpliical replacement
to plants is revealing intergroup clines. Thus Rensch (1939c)
finds a west to east increase in the divided condition of the leaves
in a Rassenkreis of the pasque-flower, Pulsatilla.

We may next consider ecological clines (ecoclines). In general,
as already pointed out, these will tend to be repeated, with
variations in slope, form, and extent, in numerous regions of a
distribution area. The increase of shell-thickness with aridity in
land snails appears to be one such example (sec instances in
Rcnsch, 1932). There appear to be numerous examples of alti-
tudinal clines, notably in size, in bird species; see Chapman and
Griscom (1924) for wrens {Troglodytes), Danis (1937) for wood-
peckers {Picus), Dementiev (1938) for various genera, and Mayr
(1931-40, No. 41) for the honey-buzzard {Henicopertiis).

Schmidt (1918) demonstrated a gradient in number of vertebrae
in the sedentary fish Zoarces viviparus in various Norwegian
fjords, the number decreasing with distance from the open sea.
Vertebral number and other characters in fish appear often to
show a broadly graded distribution (Regan, 1926; Hubbs, 1934).

In man y plants, very short ecoclines may exist. Gregor’s
investigations on Plantago maritima (1938a, 1939 ) indicate that
these are produced anew in each generation by selection from
among a wide range of ecotypes present in the species — an
important general conclusion. The differences involved may be
considerable; thus scape-length runs from just above 20 cm. in
waterlogged coastal mud types to nearly 50 cm. in those from
maritime rock. In addition, large-scale geographical dines (wliich
Gregor calls topoclincs) exist for certain characters wliich do not
show ecoclines, c.g. the ratio of scape-length to spike-length
increases from west to cast from western America (3.2) via
eastern America and Iceland to western continental Europe (4.9).

MODERN SYNTHESIS

Topoclines have been shown to exist in Pirns (Langlet, 1937) and
Iris (Anderson, 1928). and will doubtless prove a common
feature of plants as more attention is directed to the subject.

A case of abrupt steepening of a gradient is seen in the silver
pheasant, Gennaeus. The areas of two well-marked forms, one
with dark and the other with vermiculated plumage, are sep^ated
by a more sparsely populated region where no two individuals
seem to be alike. Baker (1930, p- 295) puts this down to the
rapid variation in geographical and climatic factors in the inter-
mediate region. It is more probable, however, that these are ^o
forms which have met after difierentiating in separate regions
(pp. 243 seq.) : they are in any case so different that Baker places
them in different species {Gennaeus k horsfieUii and G. Uneatus
oatesi). See also Ghigi (1909). Beebe (1921), and p. 218. Gennaeus,
as a form open to genetic analysis, merits intensive investigation.

Numerous other examples of clines of various types will be
found in Robson and Richards (1936) ; but enough will have been
said to demonstrate their widespread existence and their impor-
tance in many groups of organisms.

As Rensch points out, the various empirical rules concerning grad-
ients enable us to prophesy with considerable accuracy what will
be the appearance of subspecies from areas as yet uninvestigated.

Although some of these effects (pigmentation; altered propor-
tion of extremities, etc.), may be induced experimentally as pure
modifications, it appears certain that most of the differences seen
in nature are determined genetically. As regards their biological
meaning, while some of them, such as change in relative size of
heart, of digestive organs, and of ear-size, appear to be, in whole
or in part, direedy adaptive, many must be presumed to be
correlated with less obvious but more fundamental adaptive
changes m metabolism and activity, such as those evidenced by
the thyroid of PeromyscMS subspecies (p. 188).

It is clear that, since humidity and temperature often vary in
different ways, gradients in pigmentation will often run across
f^rh other. Doubtless many other character-gradients may run
in different directions. A. H. Miller (1931) has demonstrated this
independence of character-gradients for some characters of the

THE SPECIES problem: GEOGRAPHICAL SPEOATION 235

shrikes (Lanius) that he studied, and so has Swarth (1920) for the
fox-sparrow, Passerella (sec pp. 182, 196, 217, zzo).

It should be mentioned that geographical chnes do not always
occur. When a population is thus uniform over a large area, the
uniformity may be correlated with uniformity in environmental
conditions, e.g. in the wood-mouse Peromyscus leucopus nove-
boracensts (Dice, 1937).

Again, marked gradients sometimes exist for some characters,
but not for others. Thus in South American wrens {Troglodytes
muscuhs). Chapman and Griscom (1924) find a distinct increase
of size with altitude (doubtless a temperature effect), but little
correlation of colour with any environmental factor. This latter
fact they put down to the supposedly very recent date of the
extension of the species over the continent (cf. Heodes in North
America: p. 217). If so, then selection for increased size in low
temperatures must be more intense and therefore more rapid in
its effects than selection, e.g. in humid areas,, for whatever
characters produce changes of coloration as their correlates.

It should be mentioned that Reinig (1939) has criticized
Rensch’s views as to the adaptive origin of the chnes connected
with the Geographical Rules, and substitutes a theory accordmg to
which they are due to selective ehmination of genes during post-
glacial migration from glacial “refuges”. While this explanation
may hold good for some forms, such as the red deer Cervus
elaphus, or the swallowtail Papilio machaon, it would seem cer-
tainly not to be of general appHcation. His views, however, are
another reminder that clines are of common occurrence, and
originate in numerous distinct ways.

The general existence of character-gradients within species and
groups of related species is a fact of major biological importance
which -has been fully established only within the last few decades.
As detailed work proceeds, and is backed by genetical and
ecological study, we may prophesy that the mapping of character-
gradients will provide an important method of taxonomic
analysis, complementary to that afforded by the characterizing
of named subspecies. It should, for instance, be possible to show
on a map the hnes of maximum change for different characters.

226 evolution; the modern synthesis

If the extreme values for difierent populations of die species are
designated o and lOO, the plotting of intermediate values (phen^
contours or isophenes (p. 104) will give a contour map o t e

character-change. _ j- ■

Such mapping will obviously permit of important smdies m
comparative systematics — the determination of regularities^ and
differences in the correlation of character-^adients widi environ-
mental gradients, the tendency for subspecific boundaries to ocrar
in certain regions (Reinig, 1938; Grmnelt, 1928), the relative
variabihty of different species, and so forth. The specification of
inter-group clines will permit biologists to obtain a much clearer
picture of the mter-ielationships of the subspecies of a polytypic
species, especiahy when (as wiU probably prove to be the rule)
clines for different characters run in different directions.

In most cases, clines should be employed as a terminology
which is purely subsidiary to that of the trinomial naming of
genera, species, and subspecies. The description of clines can
provide a clarification of the taxonomic picture, as well as greater
detail of analysis, but must foUow and supplement the description
of species and subspecies, not m any way replace it. Occasionally,
however, clines must be regarded as taxonomic categories m
own right, to be employed as part of the nomenclature, in
place of subspecies. This will be so when a well-marked gradation
of characters extends without sharp break over a considerabk
area, as in the nuthatches mentioned on p. 219. Loppenthin, it
is true, assigns subspecific names to arbitrary stages in the dine,

but this would appear to be quite indefensible.

Subspedes, by definition, should mean something of the same
general nature as spedes—i.e. unique groups, with definite
characters shared by the whole population, and definite areas of
distribution; the distribution may be either geographical or
ccolo^cal. Clines, on the other hand, may be repeated a number
of times; and even when they have a definite single area^ of
distribution, by definition show a gradation, not a uniformity,
of characters. It is suggested (Huxley, 1939a) that when a xline
has a large single distribution area, and thus constitutes an infra-
spccific category equivalent to a subspecies, it should be denoted

THE SPECIES PROBEEM: GEOGRAPHICAL SPEOATION 227

by a hyphenated Latin name, preceded by the abbreviation cl.
It is further suggested that where doubt exists as to whether a
series of forms represents a single internal cline or a set of sub-
species which can be arranged in an intergroup cline, they should
be provisionally named as dines. Thus in the Burmese squirrels
{Cdlosciurm sladeni) referred to on p. 219, the northern series
of forms, instead of being divided into four separate subspedes
C. s. shortridgei, fryanus, careyi, and harringtonii, as is done by
Thomas and Wroughton (on the basis of collections from six
stations only, one of which yielded types intermediate between
two of the “subspedes”!), should, pending further investigation,
be styled C. s. ell shortridgei-harringtonii.

The cline concept can also be employed in a formal sense, to
express the gradation of forms produced by species-hybridization,
even when no geographical gradation exists. Such dines have
been called hybrid dines or nothoclines by Melville (1939), and
have been used by him in his analysis of the bewildering variety
of forms found in the elms (Ulmus).

The giving of a name to a particular group inevitably tends
to endow it with greater j&xity and uniformity than may be
warranted; and if one infra-specific group be just sufficiently
distinct to merit subspecific naming, another not, the named
group will tend to be thought of as having a greater “reality”.
The employment of dines in taxonomic description will tend to
correct this, by stressing gradational changes and the orderly
inter-connexions of groups, and will help towards providing a
truer and fuller picture of organic diversity.

5 . SPATIAL AND ECOLOGICAL FACTORS IN GEOGRAPHICAL
DIVERGENCE

We may now consider in more detail the various methods by
which geographical isolation may operate. It is clear that, when-
ever the areas inhabited by different geographical groups differ
either in their physical or their biological environment, then
adaptive changes may, and usually will, occur, superposing some
degree of ecological divergence on what we may call the pure

228 evolution: the modern synthesis

eeosrapWcal, due to non-adaptive changes. In order to discuss
lei adaptive processes adequately we must antiapate some ot
the later conclusions and point out that ecological divergence
may be of three main types. There is first, adaptation to the broad
physical features of a region, including cUmate ; tbs we may caU
^odimatic. Secondly, there is adaptation to the det^ed featur^
of a particular type of habitat within a region wbch may be
called ecotapic. And m the third place there is ecobmtic adaptation,
to a particular mode of life withm a habitat.

b ecological divergence, adaptive differentiation is prmary,
whereas b geograpBcal divergence, spatial separatton is primary.
Naturally, there al many borderlme cases; but the distinmon is
often a real one. Ecological divergence may be superposed upon
geographical, e.g. m cottons of the genus Gossypium (Silow, 1941)-
In this section we shall confine ourselves to geograpbcal
divergence, where the primary factor permitting or promoting
partial or complete speciation is the spatial separation of the
groups concerned. Tbs, however, may operate in vanous ways,
(i) b the first place, geograpbcal changes may mtroduce a
discontinmty bto a previously continuous range. Tbs will ocot
when subsidence isolates groups of a land form on islands; when
elevation separates groups of a marine form on two sides of an
isthmus; when a change of cUmate isolates groups on mount^-
tops; when ecological conditions cause a discontinuity of a
necessary food-plant; or when an anadromous fish speaes

becomes land-locked m several separate lakes. _

Such barriers are non-biological accidents superposed on me
biological continuum. They may be ecologicany neutral, when
the environment is similar on both sides of the barner. If, however,
it differs on the two sides, the barrier will be ecologically sigmti-
cant, and, by preventing gene-flow, will facflitate greater

divergence than would otherwise have occurred. ^ -

The discontinuity may erect a complete barrier to the mter-
breedmg of the two groups, as with the case of the isthmus or
the lakes; or the barrier may be partial, as with a bird population
on an island close to the mamland. Divergence m small isolated
populations may depend solely or niamly on the isolation, and

THE SPECIES problem; GEOGRAPHICAL SPECIATION 229

be due to the “accidental” incorporation of non-adaptive
mutations and new chance recombinations — ^the Sewall Wright
effect of “drift”.

(2) A similar state of affairs may arise when sharp geographical
barriers, such as rivers or mountain ridges, exist ah initio in the
path of a species which is extending its range. The species may be
able to surmount the barriers by migration, but the migration
is of small extent: thus the resultant groups remain essentially
isolated, as with Partula (p. 232).

(3) A somewhat different picture is afforded by wide-ranging
species whose range is not cut op by sharp physical discontinuities.
In such cases, the whole can form a single interbreeding group
without any marked barrien, even though mere distance pre-
vents the intermingling of the remoter portions of the population.
Divergence may then occur, as with Peromyscus, in relation to the
broad features of various regions — ^humidity, temperature, colour
of background, etc. Essentially adaptive subspecies will be pro-
duced, but further divergence into full species is prevented by
interbreeding at the margins of the subspecific areas. The sub-
species may of course differ also for accidental “isolational”
reasons.

Such subspecies will remain dependent, as parts of a single
evolving Rassenkreis. They have reached the equihbrium-point
of partial biological discontinuity, whidi is maintained thanks
to the establishment of harmoniously-stabilized gene-com-
plexes in the subspecific populations, with consequent restric-
tion of interbreeding to narrow zones (p. 210). This condition,
as pointed out by Sewall Wright (1940), is the most favour-
able for the adaptive evolution and plasticity of the group as a
whole.

This type of divergence may readily be combined with the
types outlined under (i) or (2) above, and (5) below. In such
cases, full species may arise, and divergence proceed further.

{4} When a species has been widespread and becomes restricted,
or when it is very local, interbreeding between local groups
becomes reduced, and accidental divergence, fostered by isolation
and by reduction of numbers, can play a greater part.

230 EVOLtfTION: THE MODERN SYNTHESIS

(5) When ecologically very distinct regions within a larger
area are colonized, distinct subspecies or species may be formed
in each such area. We may think of woodland as against open
country, upland as against lowland, desert as against well-watered
country, sea-coast as against inland. Groups divergmg m tins way
will in general be spatially separated, but the process differs from
ordinary geographical subspeciation, as under (3) above, m
fp ri-ain important ways. In the first place, such ecological regions
may each be markedly discontinuous (e.g. regions over a certam
height), whereas those inhabited by typical geographical sub-
species are normally each weU-defined as a continuous smgle area.
Secondly, the principle of geographical replacement may break
down, distinct groups within a region being kept ap^ by eco-
logical preferences (pp. 270 seq.). Thirdly, the ecological adapta-
tion is here on the whole primary, the spatial separatton
secondary. These facts may have a further consequence, namely
that there may be relatively more zones where the two groups
may come into contact, though there will be sharper adaptive
distinctions between them. Then, as we shall see, selection will
promote barriers to interbreeding, so that full speciation is more

likely to result, 1 ■ 1

This type of divergence thus forms the transition to ecological

divergence, and is on the whole on the ecological side of the
dividing line. We shall accordingly treat of it in a later section.

(6) When the biological environment of an area inhabited by
a group is very different from that of other areas inhabited by
related groups, the type of divergence which results from geo-
graphical isolation may be quite distinct. On oceanic islands, for
instance, a very restricted fauna and flora is usually found,^ so
that selection will act in quite a different way from the original
habitat of the species on the mainland (pp. 324 scq.). The same
may apply to large and well-isolated lakes.

In such cases the struggle for eidstence will in general be less
intense, both as regards competitors and as regards enemies. This
will allow greater play both to accidental divergence and to
ecobiotic differentiation of a rather special sort (p. 325)- In addi-
tion, the environmental factors wfll often be so different that

THE SPECIES problem; GEOGRAPHICAL SPEQATION 23 1

ecoc3imatic and ecotopic divergence also will be promoted above
the ordinary.

(y) The nature of tbe group that is spatially isolated may also
play a part in determining the type and extent of divergence, in
addition to the nature of the isolation and the nature of the
physical and biological environment in which it is isolated. For
instance, migratory forms are less likely to show geographical
divergence than sedentary ones (p. 239).

(8) Next, there are the effects of migration. Sometimes we
have the simple expansion or contraction of the distributional
area of a group. A particularly interesting process is that of the
migrations of two or more distinct groups subsequent to their
divergence. A process that is in a sense the converse of (i) occurs
when geographical change, such as elevation or change of climate,
permits subspecies or species that were difierentiated in complete
isolation to meet once more (p. 243). The result will be quite
different according to whether they are or are not still capable of
breeding together. Of rather a different nature are the alterations
in range of subspecies that have always been in contact at the
margins of their areas, as a result of changing population-
pressure (p. 209).

(9) Finally, we have numerous range-changes due to human
interference, such as introductions, deliberate or accidental, of
alien types. It is probably fair to say that most biologists are
unaware of the number and extent of such range-changes now
actually in progress.

We may now consider these general points in the light of
actual exEhnples. Cases where divergence appears to be largely
non-adaptive, due either to the accidental after-effects of isolation,
or to the equally accidental initial process of colonization by a
non-representative sample, are seen in various island forms, in
the differentiation of different races of char in European lakes
(p. 177), and in the divergence of the flora of the high mountains
of East Africa. In this last case, it appears that during the pluvial
period the present high mountain forms occurred at much lower
levels and accordingly had a continuous distribution: with
increased aridity, they were pushed up the mountain-sides into

232' . . liVOIUTiOM'.:' TUB MODERN , SYNTHESIS, -

isolation. As a result the giant sesiecios, lobelias^ tree-heaths and
other plants usually differ spcdficaliy or subspedfically , from
one mountain to another. The same occurs with birds in South
America, two well-marked subspecies of the mountain huniming-
bird Oreotmchilm ckimhotazi being found on Chimborazo on
the one hand and Cotopaxi and neighbouring mountains on
the other. The ranges of the two forms are separated by about
sixty miles. A rare intermediate form, however, appears to exist
on a. ridge midway between the two: if, so, this provides a
beautiful example of -partial isolation (Chapman, 1926, p, 301).
In these cases, differentiation, whether accidental or adaptive,
appears to have occurred wiioEy or mainly subsequent to isolation,
not by initial sampling.

A similar phenomenon, here apparently altogether due to non-
adaptive subsequent differentiation (the Sewall Wright effect),
was described by Kammcrcr (1926) for lizards on isolated islands
in the Adriatic. In one case, the two halves of one island arc
biologically isolated through the isthmus being exposed to salt
spray: and the lizards on the two lialves are of a different colour.
A well-known case is diat of the special variety of lizard found
on the isolated rocks known as the Faraglioni close to Capri.
We have referred (p. 200} to the quantitative evaluation of
geographical differentiation in insular lizards recently undertaken
by Kramer and Mertens {igiZa).

The most remarkable cases, however, are those of various land-
snails in the Pacific, as described by J. T. Gulick (e.g. 1905),
Piisbry (1912-14), and Crampton (1916, 1925, 1932). We may
take Pariula on the Society Islands as an example. The interior
parts of the islands are mountainous, cut up into deep wooded
vaQcys separated by knife-edge ridges* Snails can and do migrate
from one valley to the next, but this is an occasional phenomenon
only, since they arc adapted to warm, moist conditions, and arc
normaly not found either in the dry coastal strip, tlxe cold peaks,
or the cold dry ridges between valleys. Thus the populations of
different valleys arc virtually isolated in a genctical sense, except
for rare and more or less accidental migration.

Numerous species of arc distinguished, with different

THE SFECIES PROfJtHM CECKJRAPniCAi ' SITUATION 233

degrees of intra-spcdfic variation. The most complex species
described is P. otaheitana^ with eight subspecies and their varieties
of primary, secondary, and tertiary degree. These varieties
include dcxtral and sinistral types, giant and dwarf types, and
numerous types differing in colour and form of shell.

The different cliaracters and varieties occur in difFerciit pro-
portions in different valleys, some often being wholly absent in
particular areas. In certain eases the course of migration can be
deduced. For instance the population of Fantaua Valley shows a
remarkable degree of variation, and appears to have been the
source for the colonization of a number of other neighbouring
valleys: for the populations of all of these show a reduction in
the number of ‘'unit-characters’’ as against the Fantaua assem-
blage, but each possesses a different combination of these charac-
ters. Thus local reduction of variability by colonizing through
sinaU random non-representative samples seems here to have
been an important method of increasing geographical diversity*
On the other hand, other evidence points strongly to some of the
diversity being due to the “accidental” incorporation of new
mutations or recombinations, in the populations subsequent to
their isolation.

An important feature of Crampton’s work is that he was able
to demonstrate the process of change in operation (Crampton,
1925), He himself had been collecting since 1908, and a detailed
record had been made by Garrett from 1861 to 1888. During
this period, several changes have occurred. Extension or altera-
tions of range have been not infrequent. Colour-types ai^d
giant and dwarf forms unrecorded by Garrett have become
well-established in certain valleys. Forms showing reversal of
spiral have become established in colonics recorded as exclusively
dcxtral c)r sinistral in the nineteenth century.

The largest change occurred with another species, P. darn.
Garrett described this as 'Vcry rare”, and restricted to a small
southern area of Taliiti. By 1909 it had covered almost tour-fiftlis
of the island, and both in its old and its new areas showed a much
greater degree of variation in size, shape, and colour.

Similar phciioiucua were found with P. suturdis in M(K)rca

334 EVOI-UTTON: THE MODERN SYNTHESIS

Island Here we have a curious fact. While the speaes in Garrett s
time almost exdmivdy dexttal, and remains so to^y m
its origmal irea, it becomes progressively more simstral m_ the
newly-colonized areas. (The physiological pecuhanties of smis-
trals fp. 316) may favour their spread in certain conditions.)
Again, P. mirabiUs, so rare in Garrett’s time as to have escaped
"detection by liipi, now covers quite a wide area, and ei^bits
an extremely high degree of general and local variation. Com-

oi spreaa ana amcrcuuiinuix -v,- — r -

had been continued in the further sixteen to eighteen years.

Very simdar results have been found by J. T. Guhck (1905 ; and
see Pdsbry, 19x2-14, and Welch, 1938) with the AchatmeUidlmd-
snails from the Hawaiian Islands. It appears that a sumlar, though
not quite so excessive, differentiation has occurred among the
ampLa of the mountainous islands of the Antilles (see Barbour
ml Shreve, 1937). In other parts of the world, where these
peculiar conditions favouring isolation are
geographical differentiation of land-snails proceeds along more

normal lines (see, e.g., Rensch, I 933 ?>)-

An interesting case from mammals is that of the African
cob-antelopes (Kobus) studied by Hamilton (i9i9)- On the east
bank of the Nile there is a gap between ^o ^tmct forms
(well-marked subspecies), while on the west bank the two grade
ikto each other both geographically and m appearance. The
reason for the greater isolation on the east is not dear.

The ecological and geographical factors m the (hstribqtion and
difFerentiation of birds are interestingly discussed by Palmgren

the commonest method of geographical divergence and
probably of divergence in general, is that which we may call
eco-geographical, in wWch the primary fact oppatial sep^ation
of groups is combined with adaptation to the pecuhanties o

the areas in which they find themselves. ^

We have aheady given examples of this from a mammal

* Rensch (l933a), indeed, considers this the only mport^t method of
sneciation but he fails to deal with genetic isolation, and unduly neglecK eco-
l^ical diwrgence or includes certain aspects of it under the geographical he .

THE SPECIES problem: GEOGRAPHICAL SPECIATION 23S

(Peromyscus) and a moth {Lymt.ntria). What is probably another
example comes from amphibia. Witschi (1930) has studied sex-
differentiation in the common frog of Europe. In this animal,
different geographical races have difierent methods of gonadi
development. In the differentiated races, sex-differentiation is
-clear-cut from the outset. In the undifferentiated races, all indi-
viduals develop as females until metamorphosis, after which
50 per cent become transformed into males. In the semi-differ-
entiated races all individuals start as females, but the transformation
of the genetic males to phenotypic maleness occurs earlier.

A study of types from many localities brought out the fact
that the undifferentiated races are confined to the regions of
Central and Western Europe, which were not glaciated in the
Ice Age, while the difierentiated types are found both in the
north and in Alpine valleys, with the semi-differentiated in an
intermediate zone. The divergence of the various races must then
have taken place since the end of the Ice Age. In addition, the
races show obviously adaptive difference in habits. On arriving
at a pool or being brought into tanks in the laboratory, the differ-
entiated or short-summer races lay eggs immediately, while the
undifferentiated may not lay for one or two weeks. The time-
relations of spermatogenesis are also adaptive. Witschi believes
the differences in sex-difierentiation to be determined in some
orthogenetic fashion, but they are probably correlates of funda-
mental adaptive processes such as rate of metabolism, promptness
of egg-laying, etc. Local colour-varieties occur, but have no
relation to type of sex-development. The physiological differ-
entiation of the beetle Carabus nemoralis has already been
mentioned (p. 206).

Witschi’s case is similar in its general evolutionary significance
to that of the geographical adaptation to different temperature-
conditions found in Drosophila fmebris (p. 191). It is probable
that further research devoted to this point will reveal numerous
other cases of such climatic adaptation in morphologically in-
visible but biologically important characteristics. Porter (1941)
has demonstrated dffferent egg-cytoplasm effects on early
development in two geographic races of frogs. It wil be interest-

236 evoi.ution; the modern synthesis

ing to discover whether such difFerentiation normaily shows the
phenomenon of partial discontinmty wi& relative umformity o

rha.racter over considerable areas (p. 2C^). , . ,

Baily (i 939 ) l^^s exhaustively analysed the physiologica
pecuharitils of two morphologically ^distmguishaUe^ W
Lnulations of the water-snail Limnaea columella, sndjmds that
tiiey differ quite considerably in inherent mortality and longe^ty,
S Ld ra. of grow*. One very .

under the optimum conditions provided by
one type oulv was able to grow regularly mto a large form^ ^
pectdS shape, wliich conchologists dignify as a separate variety.
Terc we hale a good example of inherent difference m dev^^
mental potentiaUty. Further research will be needed to
whether such physiological differentiation is sporadic and lo ,
or if well-marked types (physiological subspecies) extend o

^Tl^Xre isolation has enforced new habits, but not ^ yet
new genetic adaptation, is that of the situtunga antelope {Trag^
kphu%ekii) on Nkosi island in the Sese archipelago of Lake
vfctoria (Carpenter, 1925). This species is

of oaovrus swamps; but there are no swamps on Nkosi, so the
buck OT this island have become wrtually bush-buck m habits.

Mr hoofs are short, not elongated as is normal, but this is pr^
sumably a mere modificational difference in w^r;Aey do not bark
in the iJsual way, and are exceedingly tame. If the Nkosi situtunga
should eventuiy become a genetically-adapted subspec^s, we
should have an example of orgamc selection (p. 304) foUoi^g
on isolation. Somewhat similarly, the feral camels of s^them
Spain, released over a century ago, have become restricted
nursh life, and have not colonized neighbouring sandy areas

(A. Chapman and Buck, 1893). , j xt ,.1,

An exceUent example from birds is the widespread North
American shrike, Lanius hdovidams, the ‘^‘^^button and ecolo^
of which has been thoroughly investigated by A. _H. ^er
(1031). Of this species he describes eleven subspeaes, dist^-
guished by differences in colour, size, propomons and habits;
apart from correlation of colour with climate (see above, p. 213)

IBE SPECffi geographical SPECLWION 237

he finds certain features wHch appear to be definitely adaptive.
Certain subspecies are migratory, while the others are not; the
former are more efiBdent fliers, as measured by a higher ratio
of wing-length to tail-length (differences of 4 to s per cent).
One subspecies is migratory in the northern part of its range,
resident in the southern: the same type of difference is shown
here, though as would be expected the diflerences are much less
(about I *5 per cait in the ratio).

Then some subspecies inhabit more wooded country, others
more open and more arid country. The latter must fly longer
distances from perch to perch and in pursuit of prey (a deduction
checked in two subspecies by field observation). In correlation
with this they have greater manoeuvring capacity, as evidenced
by greater length of both ■wings and tail relative to total weight
(e.g. in the best worked-out case about 3 per cent longer wings
and tail ■with a 6 per cent lighter weight). The island races show
slightly reduced wings and tail, and larger feet: this is in accord
■with the character of island birds and insects in general, which
in extreme cases are ■wingless. An interesting point is that the
size of the breeding territory varies, sometimes markedly, in
different subspecies, in relation to habitat and food supply: it
would be interesting to ascertain if this is a genetic trait.

In this species Miller finds that isolation per se has little effect
compared with spatial restriction to ecologically different areas:
this is well illustrated by one of the island subspecies.

In general, he concludes that there are three factors governing
the magnitude of the subspecific differences found: first, and most
important, the degree of difference in the en’vironmen>; secondly,
thp effectiveness of isolation against interbreeding at the margins
of the area; and thirdly, the migratory or non-migratory nature
of the group and of neighbouring groups. Sewall Wright’s work
has made it clear that die size of an area also has an influence,
small size of area implying smaller population and therefore
greater scope for accidental variation. We have noted this effect
at work in mouse-deer (p. 183) and in lizards (p. 200). Stresemann
(verbal communication) has given me another example of this.
Java, Sumatra, and their outlying small islands were all isolated

238 ' ETOLUTIOH: THE MODERN '5

from the mamland and from each other at approximately the
same time in the quite recent geological past. While numerous
distinct bird subspecies exist on the small islands, the correspon tog
forms of the large islands show no or much less divergence from
the mainland types. Here the accidental type of chmge niust be
decisive, since mere size of area should not inhibit adaptive

An important problem is raised by the empirical fact that
some species or genera show greater geograpbcal variation
than others. That exhibited by Lanius ludoviciams, for instance, is
characterized by Miher as “only moderate”. Miller and McCabe
(193 s) have stubed this question in the Lincoln sparrow {Pasmella
lincokU), which shows much less geographical dtferentiation
than ‘ its close relative the song-sparrow (P. tnehdia) and the
fox-sparrow (P. iliaca). It has only three subspecies as agamst
over fifteen in the same area for each of the other species.

Miller and McCabe reach the interesting conclusion, which
might have been deduced by the selectionist on theoretical
grounds, that this is not due to a lack of inherent variability m
the more uniform species. Its actual variability is in pomt of fact
quite high; but the variations have not been sifted out into
markedly different combinations by selection. Miller and McCabe
ascribe the difference chiefly to a difference in what must be
called temperament, P. Uncolnii tending to remain confined to
a narrow ecological niche, while the other two species are
“adventurous” in relation to range and habitat expansion, in
addition, P. Uncolnii is more migratory.

In general, ducks show comparatively Httle subspeciation. This
is correlated with a high “activity-range , as Timofeff-Ressovsky
(1940) styles the area within which individuals of a single genera-
tioD may move- For instance^ common teal {Nettion cfeccu) bred,
in England were recovered next year as far west as Iceland, as
far east as the Urals (Timofieff-Ressovsky, l.c., p. 1 12). This
species has only two subspecies, one holarctic, the other nearctic.
The name “abn%ration” has been given by A. L. Thomson
(1923) to describe northward departure in sprii^, for a new
summer area, on the part of birds which have made no eorre-

THE SPECIES PEOBEEM; GEOGRAPHICAL SPECIATION 239

sponding southward journey in the previous autumn. Abmigration
is found in other ducks besides the teal, such as mallard {Anas
platyrhyncha), tufted duck {Nyrocafuligula), and shclduck {Tadorna
tadorna) . A high activity-range must clearly be, in part, a corollary
of this habit of abmigration.

Rensch (i933<j) has approached the problem on broader lines
than Miller and McCabe. Taking Hartert’s standard work on
palaearctic birds (1903-22) as source, he has tabulated the ratio
of monotypic to polytypic species {i.e. those without and with
geographical subspeciation), and also the number of subspecies

Type of animal

Per cent

1 monotypic of
total no. of
species

Number of
subspecies per
polytypic
species*'

I. Large birds

54-5

I *6

2. Small birds, migratory

39-9

3-2

3. Small birds, non-migratory . .

29-6

7-2

4. Bats . . . . . .

82-5

2-6

5. Insectivores

71-9

3-5

^ The number of subspecies refers to the palaearctic area only. Some of the
polytypic species are polytypic when their whole range is considered, but have
only one subspecies within the palaearctic.

per polytypic species, in groups which difier in habit. He took
(i) large birds, with consequendy a greater mobility, and in
general also a smaller population-size per given area (five families
■ — ^herons, storks, ibises, bustards, and cranes— with forty-four
species) ; (2) migratory small birds (nine families, including
shrikes, warblers, thrushes, swallows, flycatchers, and wagtails,
with 288 species); and (3) non-migratory small birds (six families

240

240 evolution: THE MODERN SYNTHESIS

—crows, tree-creepers, nuthatclies, tits, wrens, and woodpeckers
— ^with 115 species). He applied the same method to flying ncrstis
related non-fiying mammals (bats and insectivores, from G. S.
Miller {1912, 1924). The results are striking (p. 239). Schaefer
(1935) shows that the distribution areas of races are much smaller
in small mammals than in birds. (See also p. 176.)

Similarly Stonor (1938) has shown that in Birds of Paradise,
the excessive development of display plumes has resulted in an
unusually high degree of geographical speciation, by restricting

FBEQUENCY OP DIFPEBENT TTOBS OF SPECIES IN HABITATS AFFORDING
DIFFERENT TYPES OF RANGE. (AFTER MAYR, 1940)

Both

continuous

and

discontinuous
(New Guinea:
290 species)

All

discontinuous
(Solomon
Islands : .

50 species)

Almost all
continuous
(Manchuria:
3:07 species)

(1) Monotypic species with
restricted range ..

(2) Monotypic species with
wide range

(3d) Polytypic species with
feebly differentiated sub-

species

Polytypic species with
marke^y differentiated
subspecies

(4) Superspedes

the power of flight and rendering the birds more sedentary. On
the other hand, the very mobile ducks {Anatidae), as we have
just seen (p. 238), with few exceptions show no subspeciation,
The same is true for other active birds such as snipe {GalUmgo,
etc.) and for the very large and mobile whalebone whales (Dis-
covery Committee, 1937). Again, degree of subspeciation is
inversely correlated with powers of dispersal in the rabbits
(Orr, 1940).

Mayr (1940) has made similar tabulations, but in this case in

THE SPECIES problem: GEOGRAPHICAt SPECIATION 341

relation, not to peculiarities of mode of Hfe, but to the geo-
graphical features of the environment (see Table, above).

In the continental area of Manchuria, where almost all specific
ranges are continuous, polytypic species are in the great majority.
The internal differentiation of subspecies, however, ri not carried
very far, doubtless because of the number of intergrading as
opposed to isolated subspecies, so that species with markedly
differentiated subspecies are only half as numerous as those with
sHght subspecific differentiation, and superspecies are very rarely
produced. Of monotypic species, those with restricted range are
very rare.

On the other hand, where an old tropical archipelago provides
the extreme of geographical discontinuity, as in the Solomon
Islands, the category of superspecies is the most abundant, and
monotypic species are not only more abundant than in Man-
churia, but it is those with extended ranges which now are
rare. The promotion of differentiation through isolation is shown
not only by the frequency of superspecies (pp. 179 n., 407), but by
polytypic species more often showing marked than slight sub-
specific differentiation. Once a superspecies has differentiated into
an Artenkreis (p. 179), its constituent forms will come to overlap,
and win be listed in one or other of the stages of a tabulation
such as Mayr’s.

New Guinea, where islands and mountains introduce a con-
siderable degree of range-discontinuity, provides an intermediate
picture. The only exception is the slight excess of markedly
polytypic species over that seen in Manchuria: however, in the
ratio between markedly and feebly differentiated polytypic
species. New Guinea preserves its intermediacy.

Such work is an important contribution to the as yet embryonic
science of Comparative Systematics, which is undoubtedly
destined to yield results of the greatest importance for general
biology.

The same type of conclusion, though not expressed in numerical
terms, has been arrived at for the fish of American rivers (Thomp-
son, 1931). Large, strongly-swimming and. actively migratory
species of fish show great uniformity of character. Smaller fish

242'' evolution t .THE MODERN. SYN.THESIS

sW much greater diversity when populations from ^erent
localities are compared, and die diSerences are greater w^en sue
species are restricted to small head-water streams than w en ^
occur in streams of all sizes. The differentiation is not gra e
along the course of the rivers, but is random, presumably a result

of the Sewall Wright “drift efect. j r

Although, as we have said, the most frequent mode or geo-
graphical differentiation is broadly adaptive, here are many cases
in which apparently non-adaptive differentiation has occurred,
cither predominandy or superposed on a general adaptive diver-
gence, or as a correlate of invisible physiological adaptation.

The diversification of the Hawaiian land-snails and probably
that of the Galapagos ground-finches appears to be largely
“accidental” in the biological sense. The colour polymorphism
of various Peromyscus races (p. 189) shows that colour in thrae
forms is not always of direct selective value. The markings of the
local species (or subspecies) Bmrremon inornatm (p. 199) appear
to be non-adaptive, and in any case show no intergradation with

the normal type. . . 1 • r

There are quite a number of cases in which subspecies or a
Rassenkreis, or geographical species of an Artenkreis, show sharply
contrasted colour-distinctions which are apparendy non-adaptive
and mutational. For instance, the northern and southern Indian
robins {Thamnobia), which do not appear to interbreed in their
zone of overlap, are sharply distinguished by the colour (brown
versus black) of the back of the males (Dewar and Fimi, 1909,
p. 378). Chapman (1927) and Stresemann (1923-6) give other
examples* Spatial isolation, we may say, permits a varying
degree of accidental divergence to be superposed on the complex
geographical grid of broadly adaptive character-gradients.

A number of different effects are aE illustrated by the fauna of

the Galapagos (Swarth, 1931. 1934; Lowe, 1936}. Here, the

mocking-birds (^Mtniidae) and ground-finches {Gi’ospizidae) illus-
trate the extreme of mere isolational divergence, while in the
latter the release from competition has permitted what can only be
described as an abnormal variability and multipHcity of forms
(p. 326). The land-tortoises also iUustratc isolational divergence.

THE SPECIES problem: GEOGRAPHICAL SPECIATION 243

while tlieir gigantism is ecological, an evolutionary response to
island life and its absence of predators and competitors, as is the
flightlessness of the Galapagos cormorant and the genetic tame-
ness of almost all the endemic birds. The flightlessness of so many
insect inhabitants of oceanic islands is similarly an example of
ecological dilferentiation, after divergence was made possible in
the first instance by isolation; but the type of differentiation is
here more directly in relation to the physici than to the biological
environment, winglessness in insects constituting an adaptation
to prevent being blown out to sea.

Thus while geographical divergence always depends for its
initiation on spatial isolation, it may subsequently be linked in
varying degrees with ecological divergence of an adaptive nature,
and also, in small populations, with non-adaptive divergence due
to the genetic accident of “drift”.

6. RANGE-CHANGES SUBSEQUENT TO GEOGRAPHICAL
DDFEERENTIATION

As geographical changes may isolate groups and thus permit them
to diverge, so, after a certain degree of divergence, further
geographical changes may permit the differentiated groups to
meet again. (We are using the term geographical in the broadest
sense, to denote climatic changes as well as elevation and subsi-
dence or physiographic alterations.)

This phenomenon appears to have had very widespread effects
upon existing forms, as we should expect from the rapid
changes of chmate and of sea-level that have occurred since
the beginning of the Pleistocene, and stiU mote those which have
taken place since the end of the last glacial period, some 30,000
years ago. Some of its results are at first sight very surprising, hi
what follows, we shall consider not only eco-geographical
divergents, but also those produced in relation to regional ecolo-
gical (ecocHmatic) diferentiation, since the effects of subsequent
migration are essentially similar in both.

M the first plac^, range-changes may bring together end-
members of a chain of subspecies. A striking example of this,
cited by Rensch (1928, I933<j), concerns the great tit {Parus

344 evolution: THE MODERN SYNTHESIS

major). There are three main groups of subspeci^ of ^iarge
Rassenkreis, extending from west to east across the_ or

-the nmjor group in Europe and Western Asia, thcbokhar^sts
group from Persia to Malaya, and the minor group from China
to Tapan: each is well characterked, but the central hokharemis
type ktergrades with both its neighbours along broad margmai

zones at either end of its range. j r » >t.

However, the western or major group also extends tax to ttie

eastwards, along a strip north of the «eas of the other two
groups and quite separated from the bokharensis group by desert
and mountain regions, and finally overlaps with the area of t e
minor group near its northern boundary m the Ainur region.
This eastward extension doubtless is secondary and has only
become possible through the amehoration of cHmate since the
end of the last Ice Age. However, where the major md tnmor
groups meet, they do not interbreed, but live side by side as
perfectly distinct “species”. Nothii^ could better illustrate the
relativity of the terms species and subspecies. Rensch also pomts
out that the end forms of a chain of subspecies may be much less
alike than good species living side by side.

Again, Larus argenuttus (Stegmann, 1934 ; Schweppenburg,
1938) forms a circumpolar chain of subspecies. But the l^errmg-
gull (L. argentatus sensu stricto) now lives inW.Europe as a good
'species side by side with the lesser black-backed g^ ( L. Juscus )
though occasionally interbreeding. The two differ markedly m
temperament as well as appearance (Richter, 1938).

An equaUy good case is that of the buck-<ye butterfly of
America, /uwonifl lavinia (Forbes, 1928, 1931)- This species shows
marked geographical subspeciation. There are three ^ m^
groups of forms— North American, Central American (mcludmg
a northern strip of South America), and South Amencm. These
intergrade at their boundaries. However, die island of Cuba is
inhabited by two types between which intergradation does not
occur, and which do not appear to interbreed. Among several
other distinctions, these differ in the presence or absence of a red
sehiicircle round the upper “eye-spot” of the hind-vmg, a
character aagnostic of all the North American group of forms.

THE SPECIES PEGBLEM: GEOGRAPHICAL SPECIATION 245

Apparently the island has been colonized from the north by the
North American group, and from the south by the Central
American group (by tv/o subspecies, which do intergrade with
each otlier on Cuba). Thus members of the two groups remain
as separate species in Cuba, while m northern Mexico they inter-
breed. Here the distribution is in the form of a chain bent round
into a circle: in the centre of the chain the two types intergrade,
but the two ends have differentiated far enough to become
biologically discontinuous. Other similar examples are given by
Rensch {1929)- Similarly the warbler “species” Phylloscopus
plumbeitarsus and P. viridams are the overlapping but non-inter-
breeding end-forms of an intergrading chain (Ticehurst, 1938).

These examples also illuminate numerous cases from Central
Europe, which on first inspection appear very puzzling, where
extremely similar species Uve side by side in the same area. The
most striking case is that of the two species of tree-creeper,
Certhia familiaris and C. brachydactyla. The latter has a longer
beak, a shorter but more bent hind claw, and is rather darker.
There are also differences in the colour of the eggs. The two
forms are so alike that their distinctness was for long disputed.
However, they appear to behave definitely as two separate
species, and not to interbreed, in spite of much individual variation
(Hartert, 1903-35). C. brachydactyla appears to be more plastic,
judgmg from the degree of subspeciation.

C. familiaris (which alone is found in Britain) is a more northern
and motmtain form, while C. brachydactyla has a more southerly
distribution; but the two overlap over a large part of their range.
The more’ northerly form alone exists in North America. This
occurs ako with the marsh and wxUow tits (p. 270), and it may
prove that these too owe their separate differentiation and later
overlap to the same causes (see below, p. 246), though their
overlap region is more extensive.

It is of interest that elsewhere one of the two species of tree-
creepers just mentioned shows an incipient stage of the same
phenomenon. Dementiev (1938) mentions that Certhia familiaris
in Persia and neighbouring areas exists as a well-marked sub-
species, C. /. persica, while to the north the type subspecies is

246 evolution; THE MODERN SYNTHESIS

found. Ill the region of the Caucasus, however, the two sub-
species have re-met, presumably after some degree of glacial
isolation, with consequent intercrossing and great variability.

Similarly, we have the true nightingale {Luscinia megarhymha)
and the northern nightingale or sprosser (L. luscinia). Although
these will cross if kept together in captivity, they remain perfectly
distinct in the region between the Vistula and the Oder where
they overlap. The yellow-bellied and red-beUied species of the
fire-bellied toad Bomhina (Bombindtor) behave in a similar way,
and so do the two closely-allied land-snails, Clausilia duhia and
C. hidentata.

The explanation of all such cases appears to be simple. In the
last Ice Age the extensions of the northern ice-sheet of the Alpine
glaciers isolated many species into a western or southern and an
eastern or north-eastern group. The exact type of separation
would have been different for different species. This permitted
eco-geographical divergence in adaptation to a nuld or oceanic
and a severe or continental climate respectively. Divergence pro-
ceeded so far that when later the ice receded and the two forms
were able to extend their range so as to meet, they did not breed
together. Doubtless this failure to cross depends mainly on
psychological barriers; the two species of tree-creeper and of
nightingale have distinctive notes. Further, the two nightingales
will mate if kept' together in captivity: none the less, they do not
in actual fact mate in nature and must therefore be regarded as
“good” though very closely-related species (see also p. 254).

Probably the common and mountain hare {Lepus europaeus
and L. timidus) differentiated in a siniilar way after glacial isolation
and were afterwards able to colonize the same areas. In this case,
however, the two species are separated ecologically, even where
they overlap geographically, the mountain hare, as its name
implies, being an upland animal as contrasted with the lowland
common hare. A fact which throws an interesting light on
regional restrictions of an ecological nature is that in hreland,
which was isolated as an island before the common hare could
reach it, the mountain hare is found in the lowlands as well as
the uplands. Habitat may thus be as much a matter of competition

THE SPECIES problem: GEOGRAPHICAL SPECIATION 247

as of close physiological adaptation. However, the Irish form has
diferentiated into a distinct subspecies, which may perhaps have
been adapted to take advantage of the habitat left open by the
absence of its competitor.

The past differentiation and present distribution of the very
distinct northern and southern forms of the water-beedes
Deronectes and Gyrinus (see Omer-Cooper, 1931) appears to be
due to the same cause. These both occur separate in certain parts
of their ranges, but intergrade in central Britain . It is interesting
that on the continent the two types of Gyrinus occur together
without intergradation, as if differentiation had here proceeded
further, to full speciation.

Other interesting examples come from monkeys of the African
rain-forest. According to Schwarz (1928, 1929), groups were
here isolated by the large inland lake that previously filled the
Congo basin. When this disappeared, they were able to meet
after previous dillerentiation. One small area of overlap occurs
between two markedly distinct types of Mona monkeys in the
Cameroons, and two similar overlap areas, one of moderate and
one of large size, between, two well-differentiated types of
Colobus monkey, each with several subspecies, in the Lower
Congo and in the forest region between Ruwenzori and the
Congo river. Schwarz puts all the Mona monkeys into the one
species Cercopithecus mona, and aU the forms of Colobus into one
species, Colohus polykomas; but since no intermediates or hybrids
have been found in these areas of overlap, it seems dear that we
should consider the diSerentiation to be of specific rank in both
cases. It is fair to state that some authorities do not agree with
Schwarz’s taxonomic groupings.

Doubtless with the progress of faunistic work, many similar
examples wiH come to Hght.

But, clearly, differentiation need not always have gone so far
as to prevent the two divergent forms from interbreedmg when
they meet again. This, it appears, is what has happened in Europe
with the subspedes of the long-tailed tit, caudatus, and

the bullfinch, Pyrrhula pyrrhula.

As regards the bullfinch, Stresemann (1919) distinguishes a

248

evouition: the modern synthesis

nortlicm and eastern form, P. p- pyrrhula, from west Siberia,
northern Russia and Scandinavia, and a southern and w-estern
form, P. p. minor, from north Italy and western Europe, inciudmg
■western Germany. In addition, there is the bullfinch population
of central Germany and the north of the Alps. Tliis appears to
intergradc with both the other forms, and shows an unusuauy
wide range of variation in size. Stresemann considers it as the
product of mixture between the other two subspecies, owr a
Load area into wliich they have re-immigrated after isolated

differentiation.* ■ -i 1 l •

The longtailed tits, Aegithalos caudatus, show similar behaviour.

On the continent of Europe and Asia there is a northern and
north-eastern subspecies, A. c. caudatus, with wlnte head md a
southern and western subspecies. A. c. europaeus, wito dark head-
markings. Stresemann {1919) and Jouard (1929) have studied
these forms. There is a broad zone in west central Europe where
excessive variability occurs, apparently due to intercrossing of die
two types on meeting.f

Stresemann also considers the cline bctw'een the eastern and
western European nuthatches (p. 219) to owe its origin to cross-
ing of differentiated types, while other authorities consider^ that
it differentiated, in direct relation -with an environmental gradient,
within a continuous population. A. H. Miller (1938, i 939 ) finds
that various “subspecies” of birds of the genus >«ro are the
product of fusion between two or more subspeaes winch have
met after preliminary differentiation; subspedfic hybridization
may here also produce striking recombinations, and small stable
populations of new type (see also p. 291 ) .

In these cases, the differences between the two groups are not
very great. In the ctows, however (Meise, 1928), the differences

It should be noted, however, that Hartert (i 903 - 35 . suppl. voL, p. 53) apgns
the mixed form entirelv to P. p. minor. Tliis may make for systematic convemence.
but the geographical distribution suggests that Streseniami s view is m prmaple

'^°+*AMin it is to be noted that Kleinschmidt (1929) disagrees with this con-
clusion, and considers that the species as a whole is very variable and “at the
mixed race does not show abnormal variability. This only shows how hard it
is to arrive at final decisions except in clear-cut cases such as the crows and
flickers (see below).

THE SPECIES problem: GEOGRAPHICAL SPEQATION 349

are-stxiking, the carrion crow, Corvus c, cor me, being entirely
black, while the hoodie crow, C. c. comix, has a light grey mantle.
So distinct are they at first sight that many ornithologists (e.g.
Hartert, 1903-35. suppl. voL, p. 6) still prefer to regard them as
full species. It should be noted that if they are to be regarded as
subspecies, then we must introduce a still further category, since
each of them shows 'definite geographical differentiation into
“regional races”. According to Meise, they exhibit no essential
differences in behaviour, voice, or ecological preferences, and
should therefore be better regarded as subspecies. In any case,
where their breeding ranges overlap, they interbreed, and the
hybrid population shows what appear imdoubtedly to be the
results of mendelian recombination, the offspring of a siagle
pair often differing a great deal in regard to the amounts and
distribution of black and grey. The geographical distribution of
the two forms is at first sight curious, with three zones of inter-
breeding, as defined by field observation in the breeding season
of birds of obviously mixed origin: one of these runs across
central Scotland; a second from near Genoa, along the south side
of the Alps, and then passing northwards to reach the Baltic in
eastern Schleswig-Holstein; and a third in Asia from near the
mouth of the Yenisei, southwards to the Altai, then south-west
and west towards the Aral Sea. The total length of these zones
is over 5,000 km. and their average width quite narrow, from
75 to 150 km. (Meise considers that the breadth would prove
to be considerably greater if an intensive study of skins were to
replace field observation.) These zones, it appears, can shift their
position; see pp. 188, 209.

Since these crows appear to be ecologically dependent on the
presence of trees, it appears quite reasonable to suppose that
during the last glacial period the crow population of the Eurasiatic
land-mass was segregated into three discontinuous groups, one
in the south-west of Europe, a second in southern and south-
eastern Europe and the Near East, perhaps as far as the Caspian,
and a third probably in eastern Siberia. If we assume that the
Central group evolved the hoodie pattern, the spread of the three
groups subsequent to the retreat of the ice could perfectly well

250 evolution: the modern synthesis

bring them into contact as indicated by the present mixed zones.
The hoodie’s colonization of northern Scotland from Scandinavia
and of Ireland from northern Scotland, fits in with other eco-geo-
graphical facts (cf. the distribution of the mountain hare :p. 246).*

An almost more striking example comes from North America,
and concerns the eastern and western species of the woodpeckers
known as flickers, Colaptes auratus and C. C(^er (Allen, discussed
by Bateson, 1913). These are by most authorities regarded as
good species and both exhibit distinct geographic subspecies,
C. auratus ranges over most of the continent east of the Rockies,
and in the north extends westwards to Alaska. C. cafer is from
the Pacific coast. Between, the regions in which they are found
almost pure is a band, 1,200-1,300 miles in length and at least
300-400 in width, where the majority of specimens exhibit
characters from both species in various combinations. Some of
the characters in question are striking: for instance, quills yellow
versus red (in every case the character of C. auratus is put first) ;
male “moustache” black versus red; female “moustache” absent
versus brown; nape-patch scarlet versus absent; throat brown
versus grey; top of head grey versus brown.

The characters of the “mixed” birds, as Bateson very clearly
points out, are only explicable on the hypothesis of crossing
followed by the recombination of a number of independent
genes. Even birds from die same nest may show marked rccom-
binatory variation (as with human families).

It seems clear that the two species originally diverged at a
period when the glaciated Rockies provided a complete barrier
between them. With the regression of the ice, the two types
could meet along the zone of the Rockies, and C. auratus could
extend northward and westward in Canada until there' too it
met C. The meeting is secondary to the divergence, and the
intergradation and interbreeding have not always existed, as
seems to be the case with many subspecies of wide-ranging forms
like PeromysfHS (see also p. 291).

* Meise (op/cit) believes that the black (carrion crow) type is the later-
evolved. His reasons, however, are of no genetic or evolutionary validity, and
ius conclusion would imply the independent evolution of the black type in one
American and two separate Eurasiatic areas, which is most unlikely.

' , . THE: ..SPECIES- , Problem: geogeaphicai speciation ■ 251

later work (Taverner, 1934, and verbal information from
A. H. Miller) indicates that sporadic “mixed” birds are found
over a much wider range than earlier supposed. Taverner believes
that auratus is more “aggressive” and that its characters are
spreading westwards faster than those of cafer eastwards.

Another North American example concerns the two warblers
Vermivora pirns and V. chrysoptera. These are sharply distin-
guished by their markings; the former is a southern, the latter
a northern form. These show a inixed zone of interbreeding at
the junction of the ranges from northern New Jersey to the
Connecticut Valley, and casually to eastern Massachusetts; here
a wide range of segregants occurs. It is interesting to note that
intergrading and segregation also occur in regard to the songs
of the two forms. (Chapman, 1924; Bateson, 1913.) Here the
history of the two forms and the reason for their initial separation
is not so clear (see also p. 254).

Chapman, though stating that no ornithologist would question
the specific distincmess of the two warblers and the two flickers,
points out that in notes and habits the flickers are very much,
the warblers fairly alike — ^i.e. no or slight psychological barriers
to mating have been developed. This is in contrast to the eastern
and western meadowlarks Sturnella magna and S. neglecta. Here
differentiation has given rise to quite unlike calls and songs, and
where the two overlap after coming together again subsequent
to the Ice Age, they do not form a zone of general mixture,
though occasional intermediates, apparently due to sporadic
hybridizing, do occur.

Another clear-cut case is that of the grackles . (Quiscalus) in
eastern North America (see Chapman, 1936, I 939 » i94o).
Chapman now regards all the forms as subspecies of one
species, Q. Apparendy two populations were isolated

during the glacial period, Q. q. aeneus in south-eastern Texas
and Q. q. quiscula in southern Florida. The latter, in its post-
glacial spread to the north and west, has di&rendated into a
further subspecies, Q. q. stonei. The western form appears to have
extended its range more rapidly, now being found in the northern
New England seaboard. It has met and hybridized with Q. q.

353

evolution: the modern synthesis

.simtei. over a long belt, extending from western Louisiana north-,
eastwards to Cape Cod, a distance of some 1,500 miles. The
intermixture is similar to that of the flickers, except that tlic two
parent forms arc not differentiated by such sharply contrasted
single characters, so that the hybrids present a more regular and
finely-graded scries of intermediates. The hybrid popnlation has
been christened Q. tidgwayu An. interesting feature is that the
width of the interbreeding zone increases steadily from about
forty miles in the south-west to ahnost two hundred miles in the
north-east. This, it may be suggested, is a time effect. The hybrids
may be presumed to be at a slight selective disadvantage as com-
pared with the pure parent forms, wiiich would lead to a restric-
tion of the hybrid zone. But the two types must have met earliest
in the south-west, so that selection has not operated for so long
in the north-cast (see p. 287). A curious minor point is that at
the eastern end of Long Island, the great range of hybrid variation
is absent, and 90 per cent of the population are sliarply inter-
mediate. (For chickadees, see p. 180.)

The red-tailed hawks {Buteo borealis) o(N. America (Taverner,
1927) present an amazingly complex picture. Two main sub-
species exist, in the east and die west respectively, die latter
diphasic and also very variable. Bodi show what Tavenicr con-
siders incipient geographical differentiation in certain regions.
Ill the north-west, presumably by post-glacial rangc-cliangc,
both the mjyor and both the minor ty^‘s have come to overlap
and interbreed, giving profuse recombination. Finally the bird s
nomadic habits appear to disseminate individuals far from their
original home (see p. 355); The species would repay exhaustive
investigation.

Dementiev (1938) gives numerous examples from eastern
palacarctic birds. In the slirikc Lanius coUurio in particular, there
exists a large region peopled by hybrids between L. c.collurw
and L. c, phoenkuroides,

Stuart Baker (1930) gives similar examples in pheasants.
Bateson (1913, p. 160) cites further examples in birds and butter-
flies, namely die zones of hybridization between two distinct
species of roller {Coracias) in India, and between two very distinct

THE SPECIES problem: GEOGRAPHICAL SPECIATION 253

species of white admiral {Limenitis or Basilarchia) along the quite
narrow line of jimction of their ranges.

In mammals similar phenomena occur in the hartebeest ante-
lopes (Alcelaphus) in the rift valley region. Ruxton and Schwarz
(1929) give graphs which show that the hybrid forms exhibit
bimodal frequency curves, as would be expected, for certain
characters. Banks (1929) gives facts which support the idea of
hybridization among certain monkey species in Borneo. Here the
different forms are separated altitudinally.

In a considerable area of the north-central U.S.A. (Sweadner,
1937), the whole population of the moth Platysamia appears to
have been produced by hybridization between two distinct
species, which again have met owing to post-glacial range-
extensions after differentiating during the glacial period. The
area here is a triangular one, expanding towards the north. The
characters of the hybrid population appear to be graded: if so,
we should then have zgenocline, dependent on a balance between
two opposed streams of gene-flow.

Dr. A. P. Blair (1941^) has investigated similar though more
complex phenomena in the toads Bufofowleri and B. uwodhousi
in the U.S.A.

A case from butterflies that seems very similar to the flickers
is that of Aricia a. agestis and A. a. artaxerxes. After die Ice Age,
the two must have met along the coast of Northumberland and
Durham. Here marked segregation occurs, giving striking
recombinations along a rough genocline (Harrison and Carter,
1924). It is imcertain where the two subspecies originally differ-
entiated. Harrison and Carter suggest Ireland for artaxerxe s, but there
is no evidence for its occurrence there to-day (Donovan, 1936).

A somewhat different phenomenon is recorded by Carothers
(1941) for two species of North American grasshopper, Trime-
tropis dtrina, a form from sandy river banks, and T. maritima
from coastal sands. Both of these arc remarkably constant in
their characters. An intermediate and very variable form has
been described from the north shore of Lakes Eric and Ontario,
an area separated from the range of either pure species. Carothers
has now synthesized all known variants of this in the F2 and

254 evolution; the modern SyNTHESIS

backcrosscs from crosses between the two pure species. It may
be hazarded that the Great Lakes form has been produced by
hybridization, but at some earlier date when conditions were
different and permitted the two pure species to meet and cross
in this locality. When conditions changed, the hybrid form must
have been able to maintain itself in this locaHty, while the others
were compelled to retreat. This, however, is purely speculative,
and further investigation of this peculiar case is dcsirab 'y

l a c k (i94of») has an interesting discussion of the role of habitat-
preference in speciation, which, in addition to its intrinsic interest,
has a bearing on some of the problems we have just been dis-
cussing. The origin of a marked difference in habitat-preference
must be, in his opinion, due to historical accident— e.g. through
a sroup of a woodland species being isolated in a re^on with
only one particular sort of woodland available, and its behaviour
then becoming gradually adapted to this type of habitat. He
comes to the conclusion that habitat-preferences are not of sigm-
ficance in originating the isolation leading to speciation, but that,
once evolved, they may play a part (together widi m^g
reactions) in maintaining the distinctness of forms which have

re-met after differentiation.

Thus the nightingale and sprosser (p. 246) not only diner m
their songs, but frequent quite different habitats, dry and ve^
damp woodland respectively. These behave as “good secies ;
but in the case of the bullfinches and the longtailed tits (p. 248)
the habitat-preferences of the two groups have remained simila^
and they have therefore remained as subspecies, and interbreed
where they have come to overlap.

The North American warblers of the genus Vermirora (p. 251)
intercross regularly where they overlap, in spite of considerable
dififcrences in plumage and song. Their habitat-preferences isolate
them partially, V. chrysoptera preferring higher slopes, but they
overlap considerably. If their habitat-preferences had differentiated
somewhat further, they would not have had the opportunity o
intercrossing. We shall meet with similar cases in mammals
(see pp. 271,283-4)-

Differcntiated forms may come to occupy me same area not

THE SPECIES problem: GEOGRAPHICAL SPECIATION ZSS

only by the process described on pp. 243 seq. but by i mmig ration
at different times. This “double invasion” (Mayr, 1940) is com-
monest on oceanic islands. However, the weasels {Musteta)
probably carried out a “double invasion” of S. America (E. R.
HaU, 1939); and the case of Pams major (p. 243) is similar. If
sufficient difierentiation has occurred between the waves of
immigration, the forms will behave as “good species’ ’, like the two
chaffinches on Teneriffe, Fringilla coetebs canariensis and F. teydea,*
or the three species of white-eye (Zosterops) on Norfolk island, if,
on the other hand, difierentiation has been slight, the phenomenon
will not be noticed, as the new immigrants will blend with the
old. If it has been of moderate extent, obvious hybrid populations
will result, as in a species of brush turkey, Megapodius {Mayr,
1931-40, No. 39).

It is interesting that in certain cases the zones of interbreeding,
notably in the crows, are so narrow and apparently so stable in
position. That of the flickers, on the other hand, is much wider
and its width is quite possibly still increasing. It is clear that a
theoretical analysis of the genetical problems arising from the
meeting of two distinct types capable of free interbreeding would
be of great interest.

In general, we should expect that the development of regionally,
stabilized gene-complexes, together with the efiects of isolation
in accumulating differences impairing fertility or viability
(p. 360; Muller, 1940), would account for the restriction of the
zones of recombination. The greater the impairment of fertility
or viability sufered by the hybrids, the narrower and sharper
will be the intergrading zone.

According to an interesting verbal communication from Mr. J.
Dunbar of Spynie, Morayshire, fifty years ago all the breeding
crows near Elgin were hoodies. Soon after this he remembers the
first nesting of the carrion crow in the district. To-day the
breeding birds are all carrion crows, implying that the zone of
interbreeding has moved north-westwards. If so, this would

* These two forms are of further interest, since both, in correlation with the
oceanic climate, show similar colour-changes, the later iminigrpit (the subspecies
of F. coekhs) to a lesser degree than the earlier, which has had time to achieve fuii
speciation (see Meinertzhagen, 1921).

2S6 evolution: THE MODERN SYNTHESIS

indicate, first that the carrion crow in Scotland enjoys a slight
selective advantage as a breeding species over the hoodie, and
secondly that these zones of hybridization, as suggested for the
genetic^y similar zones of intergradation between contiguous
subspecies, may alter their position without losing their sharpness.

A curious case is provided by the sparrows (Meise, 1936;
verbal information from the late F. C. R. Jourdain). Without
going into detail, we may say that in Spain the house-sparrow
(Passer domsticus) and the related P. hispaniolensis exist as well-
defined and well-localized species, the former near human
habitation, the latter in open country. On the other hand, in
North Africa and also in parts of the Near East the two are found
together, interbreed, and produce every kind of intermediate
over considerable areas. The plausible suggestion has been made
by Klcinschmidt that P. hispaniolensis is an original inhabitant
of the countryside in Spain, while P. domesticus is a later immigrant
and, being more of a parasite of man, has there remained more
urban. In Africa and the Near East, on the other hand, he suggests
that both arrived more or less simultaneously and began to cross
at once, before ecological segregation occurred, and that the
mixed types show no sharp habitat-preferences. In any event we
have the interesting phenomenon of two good species differ-
entiated in different regions but intercrossing freely w'hen brought
together by circumstances (sec p. 358).

The two common species of rat provide interesting examples
not only of migration after initial differentiation but of furdier
differentiation consequent on migration. A useful summary is
given by Hinton (1920). The black rat (Rattus rattus) was origin-
ally a more or less tree-Hving animal, yellowish or reddish-
brown above and white below, from India, Burma, and neigh-
bouring regions. The brown rat (R. norvegkus), on the other
hand, had its original home in Asia north of the great mountain-
chains, and is typically grey or brown above, with a silvery-grey
belly. The two may be regarded as having originally been
differentiated as mutually-replacing species of a geographical
subgenus (Artenkreis).

To-day the black rat is found in three main varieties or sub-

THE SPECIES problem: GEOGRAPinCAi, SPHClATfON 257

spc'cics: the roof rat, wliicli is the typical original form, character-
istic of India and the Western, Mediterranean; the Alexandrine
rat, a darker form, with brownish-grey back and dingy belly,
characteristic of Asia Minor, North Africa, and certain fiKliaii,
provinces; and the true black rat, characteristic of cold-tcinpcrate
Europe, with black back and smoky-grey belly, A black variety
of R. noTPCiJiais is also known, this too from Western Europe.

Commerce and navigation have carried both species all over
the world. The black rat was probably not introduced into
Western Europe until the Crusades, the brown rat certainly not
until the eighteenth century. The competition between the two
types is complex, the brown rat being more vigorous but more
dependent on water, while the black rat is favoured by warmth
and by environments where climbing is needed. Thus the latter
remains the dominant species in countries like India, and also
on shipboard, but the brown rat has almost completely ousted
it from temperate countries. However, of recent years die black
rat is obtaining a foothold in temperate ports, where it obtains
access from ships and where modern tall buildings put a premium
on climbing ability, while also discouraging the brown rat.

Perhaps the most interesting fact concerns the development of
the colour varieties. It seems clear that in the rats (which thus
constitute an exception to Glogcr’s rule), lower temperatures and
indoor life both favour darker coloration, and that the Alexan-
drine rat differentiated from the original R. rattus in moderately
cool regions, die true black rat in still cooler climates. The full
black form appears to have differentiated in Western Europe by
1530 — i.c. in less than five hundred years from its first intro-
duction. Once differentiated, the three forms (one may perhaps
call them subspecies, though of a rather unusual type) have been
still further disseminated by man, and mixed groupsof two or
of all three types may occur; incndcliaii segregation may then
be found in a single litter.

Most striking is the fact that a black variety ot the brown rat
has differentiated witliin die last two hundred years. First described
from Ireland in 1X37, it has since become commoner and has
extended its range in Britain. In E. Africa, Raitus [Maskmiys)

„8 IVOiniOH: THE MO0EEK SVNIHBEIS

has r«ndy proauced ^ut ojy in mcden.

'’’SfuSv ^ p’r "I

dn^to rigraiin hringmg difcentiaied forms togete, A good
oXlTh that of the faapweods Ce«mmjace. and C. mgra m

s (Mat^f-^ L"t7tmr;“

ittenL crossing has taken place, producing
vanous loc , of kinds, sometimes to the

segregaiits ^ eliminated, and only

i^es rauin, in various recombinadon and degjs of aun^.

S (ipap; and in Watson md oiers. 1936) m« the
faJ to *c deforestation of the Balkmr Pemnsnla by hn^
a^cy has not only enabled m«.y non-forM plants to^earad
Jeir^ge, but has &e.p»tly permitted nnnrerons spe^ that
were orkinally diftrendated in separate areas to meet, and so
I hybrifa tdth *e ptodncdon of a wedth of .jew phenotypes
whii are the toe of the notonomsst but make matenal for

“AWhS^ of aossing after originally separated types had
been bton^t together, in this case wholly by hmnan agency,
is aflbrded by the introduced flora of New Zealand. In the new
environment, hybridiaadon has occurred on a considerable scje
(cf. the sparrows mentioned on p. 256). Here, however, the
Lidon is compUcated by ihe existence of even more wi^prto
hybridization among the indigenous flora (see p. 355 ). a tact
for which it is difficult to give any satisfactory explanation unless
it be correlated with low intensity of selection by herbivores.
Similar cases in animals, but concerned with subspecies, have

already been referred to (p. 248). , . 1 1 u

Besides extensive range-changes of this type which alter^ the
degree of isolation, contact, or overlap between forms, we have
those which are concerned merely with the extension or retracnon
of areas of forms wMcli remain throngiiout in contact with each
other. Wc have already discussed these in Peromysews subspecies
(p. 208), and shown that they are probably dependent on varia-
ft tions in population-pressure. Such occurrences are doubtless

THIi SPECIES' problem: GEOGI^APHI^^^ SPECIATIO'N 259

widespread and when they occur will reduce the closeness of
adaptation between subspecies and habitat.

Migration and changes of range have unquestionably been
extensive in periods ot rapid climatic change like the recent past,
and bring many complications into the field of systcinatics. In
some cases we can deduce what has happened ; but in others we
must remain uncertain. It is, for instance, theoretically possible
that many cases which we shall discuss under the head of ecological
(ecociimatic and even ccotopic) divergence arc in reality due to
geographical isolation, followed first by regional adaptive
differentiation, and later by migration. In any event range-change
has often been extensive and important, and has contributed to
systematic diversity both by introducing pairs of species that
normally do not cross (such as the European trec-crccpcrs or the
major and minor forms of great tit) to countries where, apart
from such isolation, differentiation, and reunion, the ecological
niche would have been filled by one species only; and also by
allowing dijfFcrcntiatcd subspecies to cross and so to produce a
wide range of new segregant recombinations.

7. THE PRINCIPLES OF GEOGRAPHICAL DIFFERENTIATION

Out of the accumulation of taxonomic and micro-evolutionary
data, illustrated by genetic and mathematical theory, certain
general principles of gcograpliicai differentiation arc now emerg-
ing. First, isolation is per se a cause of differentiation (Muller,
1940). Tliis is due to the nature of the evolutionary process, which
proceeds by the presentation of numerous small mutative steps,
and the subsequent incorporation of some of them in the consti-
tution by selection, or in some eases by Scwall Wright’s “drift”.
The improbability of the mutative steps being identical in two
isolated groups, even if they be pursuing parallel evolution, is
enormously high, so that reproductive incompatibilities will in
the long rim automatically arise between them. If the direction
of selection differs for the two groups, visible divergence will
also automatically result, even in the absence of divergence due
to drift.

26o evolution: the modern SyNTHESIS

Secoadiy, non-adaptive or accidental difeentiation may occur
where isolated groups are small. This ‘'drift”, which w'e have
also called the &waB Wright phenomenon, is perhaps the most
important of recent taxonomic discoveries. It was deduced
mathematically from neo-mendeHan premises, and has been
empirically confirmedbothingeneralandin detail (pp. 58,200, etc.).

Thirdly, and almost equally important, there is the principle
of stabilized gene-complexes. R. A. Fisher’s extension of the
theory of genic balance enables us to deduce that we may expect
to find, in addition to the complete biological discontinuity
exhibited by species, a condition of equilibrium which may be
called partial biological discontinuity. In this condition, which will
occur in populations spread continuously over a large region,
groups showing relatively uniform characters over a relatively
large area will be separated by narrow intergrading zones where
interbreeding occun. This is the condition actually found in the
subspccific differentiation of many forms. The existence of partial
geographical or ecological barriers will prom.ote and accentuate
this type of subspcciation, but it may occur even in their absence,
provided that the region concerned is large enough, and that
there is enough ecological difference between different areas
witliin it (p. 208).

As corollary to this, it becomes clear that geographical sub-
species are of two biologically distinct types — ^those that may
differentiate into full species, and those that, unless geographical
or climatic conditions alter, wiU remain as interconnected parts
of a polytypic species. We may call them independent and dependent
subspecies respectively.

A further corollary is that, since the intergrading- zone is
automatically kept narrow by selection, dependent subspecies may
be maintained in spite of considerable range-changes : the areas
of the different subspecies may expand or contract, but the sub-
specific groups will maintain their distincmess and the intergrading
zones will remain narrow.

Founhly, we have the phenomenon of graded differentiation,
which may be subsumed under the head of dines. A priori selection-
ist considerations would suggest that, wherever environmental

THE "SPECIES problem: . CEOCRAPinCAL SPECIATION 261

agencies vary in a graded way, organic variation would be forced
into a correspon,dirig gradation. The matter, however, is not so
simple. It is complicated by two facts — the fact of partial
biological discontinuity just discussed, and secondly the fact
of migration and range-change. The fact of partial biological
discontinuity prevents the realization of a continuous gradation
of characters running paraUel with the environmentai gradient,
and substitutes a staircase or stepped ramp for a uniform slope.
The mean values for the environmentally correlated characters
of the treads of the stair— the subspecies — then show gradation,
and constitute an external or inter-group dine. This type of dine
has been the subject of the '"geographical roles’’ of Glogcr, Allen,
etc. The gradation within the several subspecies has been much
flattened, and in most eases still awaits empirical verification;
when present, it constitutes an internal dine.

It is important to note that dines for dilFcrcnt characters may
run in dificrent directions. Thus specification by dines permits
the construction of a new and more complete picture of variation
within the species.

The above statement applies mainly to animals. In plants,
broad geographical dines of this type appear to be absent or
subsidiary, while there is a much greater prevalence of less
extensive ccoclincs, wliich come into existence by selective
elimination from a large range of genetic types adapted to
different ecological requirements. This distinction seems to be
due to the more random and broadcast mcdiods of fertilization
and dispersal found in higher plants (p. 276).

Range-changes will dearly tend to obscure the regularity of
graded differentiation. If extensive enough, they may obscure it
altogether; if moderate, they will destroy die regularity of
correspondence between intergroup dines and environmental
gradients. Where hybridization occurs, it introduces an additional
source of disturbance. A special type of range-change is the
cyclical, produced by periodic fluctuations in population-size.
These may cause die population to spill over at the inargin of
its area of distribution and then recede again (see Tiiiiofccft-
Rcssovsky, 1940).

262 EVOLUTION : THE MODERN SYNTHESIS

Fifthly, within, the large areal or regional groups of species or
subspecies, a much greater degree of localized diiierentiation
occurs than was previously suspected. Where a species is distri-
buted in isolated colonies, each colony may differ from every
other, sometimes sufficiently so to deserve the name of micro-
subspecies. But even where distribution is fairly uniform, a
surprising degree of local variation is to be found if search is
made for it, both in visible characters and in the complement
of invisible recessives carried. Such incipient geographical differ-
entiation may be more or less stable or permanent, or may
fluctuate unstably with time. When local differentiation is com-
bined witii periodic population cycles, peculiar results may occur,
periods of great variability (during recovery after a population
minimum) alternating with periods of stability, but during each
period of stability with the type showing new characters (see
p. 1 12 and c.g. Dobzhansky, 1939a)-

It is in this field, of population studies on the genetical structure
of species, that the most valuable results, for evolution as well as
for taxonomy, may be expected in the near future.

Postscript. — i. A further possible cause of geographical
polymorphism (p. 106) is mutation restricted to an isolated area.
This is found in Corpus corax varius, the Faeroes subspecies of
raven (Salomonseu, 1935). This subspecies was dimorphic, a
partly whitish form existing in addition to the normal black. The
piebald form was fairly common in the sixteenth to eighteenth
centuries, but lias now become extinct owing to the depredations
of collectors. Here mutation leading to balanced dimorphism
seems to have occurred in the Faeroes alone.

2. Hovanitz (1941) shows that pigmentary clines in butterflies
(p. 214) present “astounding regularities”, but differing for
different pigments. We have (i) melanin pigments in ail families,
(2) ground-colour pterins (orange to white) in Picridae and
Papilionidae, (3) tawny ground-colours in other famihes. To
decreased temperature and solar radiation and increased humidity
and rainfall these react thus; group 1, by darkening (increased
intensity and extension); group 2, by lightening; group 3, by
darkening in intensity, but either increased or decreased area.

CHAPTER 6

SPECIATION, ECOLOGICAL AND GENETIC

1. Local t^mwgeograpkical differentiation . . . • i?. ^63

2. Ecological divergence 265

3. Overlapping species-pairs P-

4. Biological differentiation . . . p* 29s

5. Physiological and reproductive differentiation . . . p- 30^

6. Special cases . . 3 ^^

7. Divergence with low competition; oceanic faunas . . p. 323

8. Genetic divergence * P* 328

9. Convergent species-formation 339

10. Reticulate differentiation . 35^

11. Illustrative examples p* 35^

I, LOCAL versus geographical differentiation

when we examine the question of evolutionary divergence
more closely, we shall find that two rather distinct problems are
involved. We may perhaps begin by looking at the matter
historically, and from the point of view of pure taxonomy.
Two phases may be distinguished in the history of modern, as
opposed to Hellenistic and Moorish, taxonomy. In the first,
wliich begins with Gesner, Gerard, and Caesalpinus, the primary
motive was medical. In large measure it sprang from the need
for identifying the plants prescribed in mediaeval mcdicmc.
Though reinforced and broadened, first by the emergence of
commercial seed-production for horticulture in the eighteenth
century, and secondly by the dcHberate policy of reporting on
the flora and fauna of the new English and Dutch colonics (sec
various reports in tlie Phil. Trans. Roy. Soc., New York Colonial
Documents, Hakluyt’s Voyages, the official Hortus Malabartcus
of the Dutch East India Company, etc.), its predominant charac-
teristic and achievement was its preoccupation wit 1 t ic ica
situation. The major contributions were made by naturalists
concerned with the animals and plants of their own country.

264 evolution: THE MODERN SYNTHESIS

This regional or local phase reached its climax in the work of
Morrison, Ray, and Linnaeus.

The second phase, stimulated by an intensification of the
colonial motive and the extension of horticultuFal enterprise
(e.g. the foundation of the Royal Horticultural Society by
Thomas Knight), begins with the voyages of Banks and the
collections of Raffles. Its special characteristic is the impact of the
Australian and remote Oriental fauna and flora on the scientific
consciousness of Europe. The subsequent development first of
steam navigation, and then of the new colonial policy which
followed the break-up of the great trading monopolies, and was
more distinctively national in character, conspired to produce
a new orientation, in which local preoccupations were swallowed
up in a study of broad geographical distribution. This led even-
tually to the cstabhshment of the great museums, with their
vast collections and their staff of professional classifiers and
dcscribers. This phase of taxonomy had its social roots, first
in the desire to introduce new and useful plants and animals,’
and later in the need, from the standpoint both of health and
of agriculture, especially in the tropics, for identifying disease-
bearing animals, insect pests, noxious weeds, and potentially
useful crop-plants and trees.

The first phase we may style that of the herbal, the second
that of the museum. The first is essentially regional, the second
essentially world-wide.

Now let us see what taxonomic problems emerge as a result
of these two approaches. In the regional phase, the classifier is
confronted with related species either occupying quite distinct
ecological habitats or found together over much of their range.
In the former case, spatial isolation dearly facilitates ecological
divergence. In the latter case, it is generally found that the over-
lapping species show definite though often shght ecological
dificrences in habitat-preference or in mode of Hfe. This may
give us an insight into the adaptive basis for the differentiation
of the related forms, but we are immediately confronted by the
problem of how they are kept distinct and separate in nature,
and still more how they were prevented from breeding together

SPEC! ATION, ECOLOraCAL AND GENETIC 265

ill early stages ot their divergence. A frequent feature of over-
lappiiig local differentiation is the sharpness of certain characters
differentiating the related forms.

Quite other facts confront the iimseurn systcniatist investi-
gating the world-wide distribution of a group. What strikes
him most forcibly is the phenomciion of geographical replace-'
merit, as described in the preceding chapter. Geographical forms^
whether species, or subspecies, arc normally not distinguished by
obvious qualitative characters, but by smail or general differ-
ences, in colour, size, and proportion. Where the differentiation
appears to be adaptive, the adaptation is usually or mainly to some
broad regional influence such as climate or soil; it is often physio-
logical, and then accompanied cither by no obvious morpho-
logical distinctions, or by morphological characters which are
purely consequential on the physiological adaptation, and not
themselves adaptive.

The question of preventing intercrossing between groups of
this type docs not arise, since gcograpliical separation provides
the requisite barriers. On the contrary, the major biological
problem has been that of accounting for that fraction of the
divergence which is not adaptive, and this would now appear
to have been settled in principle, as due to the Scwall Wright
phenomenon of drift.

The main preoccupation of taxonomy in the past half-ccntury
has been geographical To-day, however, now that the principles
of geographical differentiation have come to be generally recog-
nized and in broad outline understood, attention is once more
being focased upon the local situation, but in the light of the
new discoveries of cytogenetics and ecology.

2. ECOLOGICAL DIVERGENCE

We shall return later to the basic question of the prevention of
crossing between spatially overlapping species. Our immediate
problem is the study of systematic diversity wliicli is based
primarily upon ecologically adaptive divergence.

We have already pointed out that ecological and gcograpliical

266 evolution: the modern synthesis

divemcnce overlap * and have mentioned the mam different
types of ecological divergence. These may be summarized as

follows:—

1. With geographical isolation primary: ccogcograpbcal.

2. With ecological specialization primary:

(a) ccoclimatic: adaptation primarily to distmet regions,
affering in cHmatic and other general environmental

(b) ccotopic: adaptation primarily to distinct local habitats.

We have also stressed the role which may be played by range-
changes subsequent to differentiation. , , ,

Ecogcograpliical divergence has been treated at length. Where
marked climatic difference betwebn two areas is associated with
geographical barriers to migration and interbreeding, ecological
divergence will proceed more rapidly and good speaes may
differentiate in place of mere subspecies. The primarily ecological
divergences we cannot discuss in such detail, since much less is
known, or can be deduced, about their early stages. ,

The most obvious examples of ecocHmatic divergence are th<^
where two species replace each Other altitudinally within the
same main area. Examples from Britain are the ptarmigan and
the red grouse {Lagopus scotkus and L. mutus)^ the twite and the
linnet {Carduelis flavirostris and C. camahinay the ring-ouzel and
the blackbird {Turdus torquatus and T. meruk); the mountain and
the common hare {Lepus timid s mi L. europaeus) over most of
their range; the alpine and the common lady’s mmtle {Alchemilla
alpina and A. vulgaris). An example from S-witzerland is that of
the black and common redstarts {Phoenicunts ochrurus and P.
phoenkuiHs). The difference between a maritime and an inland
region provides opportunity for the same type of divergence.
Wc may instance the rock and meadow pipits {Anthus spinoletta
petrosus and A. pratensis), ot th& two species of bladder-campion,
Sikne maritima and S. vulgaris (see p. 268).

* Plate (1913) has a valuable discussion of the subject in which he treats
the different modes of isolation from a somewhat similar viewpoint.

Sl'I-CIATTON, KCOLOCICAl. AND C:I:N1-T1<; 2f)7

At first sight it would appear very difficult to maintain any
real difference between such ecoclimatic adaptation of distinct
environmental regions witliin a single geographical area, and
ccogcographical adaptation to environmentally distinct geo-
graphical areas. However, a theoretically important distinction is
. possible. It may be that in an originally continuous population,
those groups inhabiting climatically very distinct regions became
closely adapted to the conditions of those regions. If selection in
favour of such adaptation were intense, selection would also act
to erect barriers to the interbreeding of the groups, since such
interbreeding would hinder the adaptive change. There is a real
distmction between cases in which spatial isolation, brought about
by purely geographical barriers, is primary, and ecologically
adaptive divergence is subsequent and secondary; and those in
which ecological divergence is primary (even if it occur in
different regions of a range) and tends to erect barriers to free
interbreeding. This will be reflected in the distribution. In cases
where gcograpliical divergence is primary, the range of a geo-
graphical group (subspecies or species) will in general be a whole
area. Where, however, ecological divergence is primary, die
range of each divergent group wfll in general constitute a type
of regional habitat— all mountains above a certain height for
ptarmigan, all rocky coastal areas for rock pipits — ^which will not
constitute a single geographical area but will be discontinuous.

An example wliich well demonstrates the interconnection
between ecological and geographical divergence is that of the
two species of bugle, Ajtiga chamaepitys and A. chia (Turrill,
1934). A. chia is a highly polymorphic species found in the
eastern Mediterranean and eastwards into Persia, in various
natural habitats. A. chamaepitys, on the other hand, is found in
Central and Western Europe and parts of North Africa, is on
the whole very uniform, and over a great part of its range is a
weed of cultivated land. The two forms are connected by every
intergradation over a zone of very considerable width. In addition
there is a character-gradient (clinc) traversing both forms from
nordi-wcst Europe to the Near East. As one passes in this direc-
tion, the plants tend to have a longer duration of life, become

268

evoiution: the modern synthesis

more bushy in habit, with shorter leaf-Jobes and larger lowers
(the gradation reaching &om lo to 28 mm. coroUa-length), and
tend towards corrugated instead of pitted seeds; the gradient
appears to be steepened in the zone of intergradation.

Turrill concludes that the original home of the two forms
was in the Near East, and that A. chamaepitys has ^nseji *57 an
exlCTsioii Bortli-wcstwards in relation to the sprea o ^
agriculture, selection having operated to reduce the "variabihty
of the stock and to adapt it more closely to the status
of arable land. On this supposition t — ^
are not due to interbreeding between two

from the more variable form. On distribution alone the two
forms could be regarded as geograpWeal divergents, but since

A. chamaepitys is so i
ments, it seems easier to suppose
niche provided by human

factor in stimulating its - 1 .

nation could not occur in the original r^ge of the speeds, but
only where agriculture was combined with other climatic con-
A\nnn^- thus freoeraohical separation here resulted from ecological

of a weed
the' zones of intergradation
) differentiated forms,
tpleTe selective differentiation of the more uniform

iphical divergents, but since
sharply characterized in its ecological requke--
that it was the new ecological
agriculture which was the primary
differentiation. However, this differen-

SPECIATION, ECOLOCICAL AND GENETIC 269

far north as S. maritinui. Much ot the natural variation is parallel
in the two species, but certain variants jrc found only in 5.
vulgaris. It appears likely that in Britain S. markima survived the
Ice Age, while S. vulgaris was a post-glacial immigrant.

Although under experimental conditions the two species can
be crossed, and then yield fully fertile hybrids, they rarely cross
naturally in the main portion of the area of their gcograpliical
overlap. Spatial isolation, due to their ecological preferences,
thus keeps them apart, and they must be regarded as good
species. However, in some snullish northern areas the available
evidence suggests tliat the two have come to overlap regionally,
and that here they have been fused to form a still more poly-
morphic mixed population. If so, this would be parallel to die
case of the sparrows mentioned on p. 256.

The two speedwells, Veronica spicata and V. hybrida, arc very
similar in appearance, but are kept separate by the adaptation
of the former to a continental, of the latter to an occairic climate
(SaHsbury, 1939).

It will obviously in many cases be difficult to distinguish
ecochmatic from ecotopic divergence. A case in point is the
crested tit {Parus cristatus), wliich is confined to coniferous forests
and to a certain range of environmental conditions. In many
areas it thus becomes Hmited to motmtain regions, but clsewltcre
(e.g. Scotland) it descends lower. The special (ecotopic) ratlicr
tlun the general (ccoclimatic) ecological conditions of die habitat,
however, appear to be much the more important.

The evolution of the crested tit in Britain affords an interesting
contrast with that of the cole tit (P. atcr). As lias been pointed
out by Lack and Venables (1939) anti J- Fisher (i940t) both
species are typically restricted to coniferous woodland. During
the last glacial ma xim um, coniferous woodland extended across
what is now the EngUsh Channel into southern Britain, and both
species were presumably restricted to this habitat, as on the
continent to-day. Later, Britain was cut off from the rest of
Europe, and its coniferous forest receded northwards, being
replaced by deciduous woodland in the south. The British cole
tit adjusted itself to the new conditions by becoming adapted

found over die whole of Britain. The crested tit, However, was
for some unexplained reason less plastic, so that the British sub-
species is now restricted to the central highland region of Scodand.

Various complications of simple ecoclimatic divergences occur.
For one thing , altitudinal separation is, of course, often translated
into geograpHcal separation at the margins of the range, the
form adapted to high altitudes extending to low levels in regions
which are too cold for the other form. Numerous examples of
this are given in Chapman’s notable monograph (1926) on South
American bird-Efe.

A remarkable example of divergence in ecocEmatic preference
without morphological differentiation is that of the lesser white-
throat {Sylvia curruca) cited by Oldham (1932).' In Britain this
is a lowland bird, never nesting above 1,000 ft. ; but in the Swiss
Alps it is not found in the valleys at all, and breeds only above
4,500 ft., in pine forests.

A peculiar case is that of the marsh and willow tits {Parus
palustris and P. atricapittus). These are in many parts of their
range extremely simdar in appearance. The chief plumage dis-
tinction is that the black of the crown is glossy in the former
species, dull matt in the latter. The marsh tit rarely if ever
excavates its nest-hole, while the willow tit always or normally
does so. Further, the notes are distinct. The ranges of the two
Fomenkreise are by no means identical, P. palustris not being

SPECIATrON, ECOLOGICAL AND GENETIC 271

Spread extensively since their separation, and under certain con-
ditions now compete within the same area. The distinction in
nesting habits is a mark of ecological specialization: in some
areas tliis is also indicated by difference in habitat-preferences,
for instance in regard to type of woodland frequented.

A somewhat different case exists in mammals. In die long^
tailed field-mice (Apodemus) two closely-related species, A.
sylvaticus and A. jlavicollis, are generally recognized. The latter
is slightly but distinctly larger, and has more yellow on the chest
and neck. The osteology also presents some definite distinctions.
There is partial ecological isolation, jlavicollis being restricted to
woods and scrub, while sylvaticus prefers more open habitats
(Zimmermann, 1936). They are found within the same area m
much of Western Europe, though sylvaticus has a much wider
distribution. In some regions, as in Scandinavia, intermediates
are found; but here sylvaticus is a lowland form, jlavicollis an
upland form, with the intermediates found in the transitional
region (Barrett-Hamilton and Hinton, 1910 — , p. 545). How-
ever, as the two forms are traced eastwards across die Eurasiatic
continent they become less distinct, until in Eastern Asia only
a single type can be distinguished (verbal information from
Mr. M. A. C. Hinton).*

In many cases of apparent ccoclimatic divergence, we must
allow for the possibility that the differentiation was in origin
ecogeograpliical, and that subsequent migration later brought
the two forms into the same area, where, however, their different
ecological requirements segregate them into different regions.
We have given examples of this in a previous section. From
what we Imow of the common and mountain hares (p. 246),
we must be prepared for the possibility that similar cases, like
that of the red grouse (or alternatively the willow grouse) and
the ptarmigan, may be of this nature. Tlie fact is that compara-
tively little is known on the matter; and it would be extremely
valuable to be able to distinguish the results of ecogeographical

* Sviridciiko {1940) has recently shown that in Russia there is an ccobiotic
difference, A. fiavkollis consuming more green food and fewer insects than
A. sylvaticus.

evolution: the mo.beeh .syhthesis

divergence followed b/ migratioE from those' ol 'ecoclimatic
divergence wi- situ. Systematic mapping of the actual imges^o£
a number of species and subspecies as found to-day, together
with their ^ probable ranges during the last glacial maximum,
would shed much light on this problem.

■■ ■ In. general, as pointed out by Mayr (Stanford and Mayr, 1940),
higher-altitude subspecies of birds are larger and darker than
mid-mountain and lowland forms. In S. America and New
Guinea the evidence strongly suggests that most altitudinal races
have difFerentiated in situ; the types are accordingly often con-
nected by **a graded series of intermediate populations^^ In two
caa^s in New Guinea, however, the local altitudinal represen-
tatives seem to have differentiated in separate localities and to
have come into their present close proximity by subsequent
range-change (cf. the marsh and willow tits in the Alps, p. 270).
The types are then usually sharply distinct, without intermediates.
This type of origin seems to have been particularly frequent in
N. Burma.

In Nyasaland the two white-eyes Zosterops virens and Z. se^e-
gdensis die separated both altitudinally and ecotopically, the
former being restricted to the borders of evergreen forest at
high elevations, the latter occurring at lower altitudes and often
in the interior of open woodland as well as the borders of ever-
green scrub (Benson, 1941). A rather similar difference holds
for two species of Cinnyris, C. manoensts and C. zomrius, and
for two subspecies of tit, Farm n. niger 2nd P. «. insignis.

We now come to ecotopic divergence. This clearly overlaps
with' ecoclimatic.' The ecological adaptation of the rock pipit
{Amhiis spinoletta petrosas) is in one aspect ecoclimatic, to a mari-
time zone, in another ecotopic, to rocky ground (p. 279). The
other two common British pipits show ecotopic divergence, the
meadow pipit (A pratensis) being a bird of moors and rough mea-
dows, the tree pipit (A. frtuialis) demanding partially wooded areas.

A case of ecotopic subspeciation in birds where the two forms
are kept separate by their ecological preferences is afforded by
the very distinct salt-marsh and dry hillside subspecies of the
song-sparrow [Mehspiza metodia) in San Francisco Bay. The

ANJ} GEN'ETfC . ' , :273

king , rail (Rallus elegans) and' the clapper- rail (H. longirosiris) of
the U.,S.A. are restricted to fresh-water and salt-marshes respec-
tively;' here the divergence has reached species level (Example
from Mayr, 1940,)

The caribou {Rangier tarandus) exists in Canada in woodland
and barren-grouJid subspecies. The former is considerably larger,
blit has smaller antlers, in adaptation to the obstacles of its habitat.
It migrates, south in summer, the barren-ground form north.
The two are kept sharply apart by their ecological preferences.
In fish, two subspecies of bream (Abramis hrama) difier also in
time of spawning; the spring form has a much more restricted
distribution (Velikokhatko, 1941).

Ecotopic divergence seems to be considerably rarer in verte-
brates than in insects (p. 322).

Plants provide numerous examples. Divergence in relation to
the calcium content of the soil is not infrequent, leading to the
production of calcicole and caidfuge species (or subspecies).
Examples of such species-pairs from Europe include the bed-
straws Galhm saxatile and G. syhestre (the caidfuge type is in
each case mentioned first), the gentians Gentiam excisa and G.
clusii^ the anemones A. sulphurea and A. alpina, and cases from
Rhododendron^ Achillea, etc. (Salisbury, 1939). It is interesting
that the anemones are physiologically buffered against the
environmental difference in caldum-content, the ash of the
calcicole species containing slightly less Ca. In a somewhat
similar way, animal species may be buffered with regard to
temperature conditions, forms adapted to regions of higher
temperature having, at any given temperature, a lower meta-
bolism than close relations living in cooler conditions (p. 435;
Fox, 1939; Fox and Wingfield, 1937). gentians, however,
no buffering has developed, and the caldcole species contains
considerably more calcium.

Dr. Turrill tells me that an even greater number of spedes
(or subspecies) pairs are differentiated in relation to serpentine
or its absence.

The two bedstraws can both be grown in a wide range of
soils, but in nature they are rather rigidly caldcole and caidfuge

274- evolution: the modern

respectively. This is a frequent phenomenon. Many plants which
in the absence of competition show wide tolerance axe, under
the more intense selection found in nature, confined to a small
section of their potential ecological range. Edelweiss grows
luxuriantly at sea-level in an English garden: it is a mountain
plant in nature, owing to its inferior performance in competition
with lowland types, combined with its wide range of tolerance
which will permit it to grow in regions above their capacity
to colonize. (See also pp. 446-7)-

The morphological distinctions between such ecotopic species,
as between the “biological species” of various animals (see below,
p. 296), may be remarkably slight. Thus the two species of
gentian mentioned were long confused. They both occupy the
same ecobiotic niche in the Alpine pasture community, and
differ visibly only in the presence or absence of green spots
inside the corolla tube, and the mode of insertion of the calyx
teeth. Such types are normally kept so isolated by their ecobiotic
adaptations that they are properly to be regarded as species.
However, just as in certain extreme parts of their ranges the
two ccochmatic species of Silene overlap and there cross freely
(p. 269), so in exceptional circumstances ecobiotic species may
hybridize. Salisbury (1939) has shown that the two oaks, Quercus
rofcHr and Q. jcssiij/fora, arc distinguished mainly by preferring
heavy and light soils respectively (and sec also Watson, W., 1936,
J, Ecol. 24 : 446). Where soils of the tw'O preferred types meet
abruptly, a narrow belt of hybrids is found, reminding us of the
narrowzoncs ofintergradation found between many geographical
sub-spccies of animals. Where, on the other hand, soils of a truly
intermediate nature occur, an entire wood may consist of a mixed
population of the two pure forms together with hybrids.

Miller Christy (1897) has described the behaviour of the true
oxlip {Primula elatior). This species is in Britain rigorously con-
fined to the boulder-clay in its highest and most sohd areas. It
hybridizes freely where it comes into contact with the primrose
(P. vulgaris), but for the most part the two species are kept sharply
apart by their ecological preferences. Interestingly enough, in
some areas the oxlip is being ‘'hybridized out” by the primrose.

SPECIATION, ECOLOGICAX AND GENETIC 275

the zone of intercrossing apparently advancing into the oxlip
area. On the continent, the oxHp has not such a restricted ecolo-
gical preference, and is correspondingly more abundant. The
distinction of the oxlip from the cowshp (P. veris) is maintained
by hybrid sterility, not by ecological preference.

In discussing chnes (p. 223) we have mentioned the ecoclines
fomid in such plants as Plantago maritima, in which a wide range
of genetically different forms is difteraitially adjusted by selective
balance (p. 103) to different parts of a wide range of ecological
conditions. These conditions may be artificially imposed, e.g. by
the degree of grazing by sheep (see Gregor, 193 8/j).

This typical plant mode of intraspecific differentiation (which
of course may, through subsequent isolation, lead on to full
speciation) is usually manifest in a broader way, over larger
areas. Turesson especially has developed tins view. According
to him , most or at least many plant species consist of numerous
ecotypes, each adapted to a certain range of environmental con-
ditions. Usually the difierentiation is ecotopic or ecobiotic,
related to habitat conditions. To take but one example, Turesson
{1927) finds that the grass Poa alpina in Scandinavia comprises
alpine, sub-alpine and lowland ecotypes, highly selected in regard
to such features as earliness and water-requirements. From his
analysis he draws some interesting biogeographical conclusions,
e.g. that the lowland ecotypes are not glacial relics. See also
Turesson {1930) for more general discussion.

In stUl other cases, the differentiation may be of die same
general nature but with still broader bash, in relation to climate.
Numerous examples of this are given by Sinskaja (1931) for
Russian plant species. Thus in grasses like Bromus inermis there
are definite “climatypes” (climatic ecotypes) which each charac-
terize a particular cl^atic zone. “Outhers” of one cHmatype
within the main zone of another may occur, and are also associ-
ated with regional peculiarities of the habitat wliich make it
approximate to the zone normally inhabited by the “oudier”
type. Presumably, as in Plantago maritima, but over a much
vaster area, selection sifis the array of ecotypes present in the
species in accordance with the climatic and other pccuhariti^

276 E¥OLUTION :,..THE -MOPEH'M S¥N'TliE.SIS; ...

of the habitat. The difference, though of great significance, is
essentially a quantitative one.

In many forms, several ecotypes may coexist in one area, each
adapted to shghtiy different habitats. But where conditions are
exacting, only a single main ecotype may survive over a con-
siderable area (see c.g. Stapledon, 1928, on cocksfoot grass).
This multiple-ecotype species-structure of higher plants is to be
contrasted with the regional differentiation typical of higher
animals, and leading to geographical subspecies: the difference
is doubtless due to the random methods of fertilization and
distribution in plants. Another type of species-structure is pro-
vided by the existence of seasonal ecotypes, each adapted to
flowering and fruiting at a different time of year: see e.g. Clausen,
Keck and Hiesey (1937, p. 15), for the tarweeds Hemizonia.

The ecocline mode of differentiation revealed by Gregor’s
work may also, it appears, sometimes be pushed further until
the range of groups is broken up into well-marked ecological
subspecies or even distinct species. Salisbury (1939) gives two
examples, both interestingly enough from a similar range of
environmental conditions. Thus the gkssworts Salkontia dolicho-
stachya, S. herbacea {sensu strkto), S. gracillima, and S. disartkulata
are found at progressively higher and drier levels of salt-marshes.
An interesting ecotopic complication is seen in that the moder-
ately low zone is characterized by S. herbaceawhen more muddy,
but by a closely allied species, S. ramosissima, when more sandy.
A similar but less complex series is provided by the sea-lavenders,
Limonium^ (Statke) rarifiora, L. vulgare {sensu stricto), and L.
pyramidale.

Ecolc^kal succession may be the agency wliich keeps such
forms sttf ciently separate for group-differentiation and eventual
spedation to occur, one and die same salt-marsh at different
stages in its career being habitable by one only of the types.
Frequendy, however, conditions are such that two or more of
the forms are found at different levels of the same marsh, so
that it is difficult to envisage the precise nature of the isolating
barriers which must have been operative to effect spcciation.

In certain cases there may be a marked segregation of ecotypes

SPECIATION, ECOLOGICAL .AND 'GliMETIC 277

adapted to liigMy specialized habitats. Thus we have prostrate
‘Varieties’' of bittersweet {Solmmm dulcamara), broom {Cytisus
scoparius), and of the hawkweed Hkracium umhellatum, restricted
(and highly adapted) to shingle, eliffdcdgc, and shifting sand
habitats respectively. Ail three retain their characters in culti-
vation, side by side with the normal form. In the Solanum the
prostrate form has a complex miiltifactorial basis (results shortly
to be published by Mr. Marsden-Jones) ; we may expect the
breeding experiments now being carried out by Dr. Turrill to
reveal the same general state of affairs in the Cytism.

In the Hkracitmt, Turesson (1922) showed that the dime-type
occurs all along a long stretch of dunes, except near the few
spots where woodland comes down close to the dunes. He
interpreted this as meaning that in these localities, cross-pollmatioii
from the woodland form has wrecked the specialized constitution
of the sliifting-sand form sufficiently to prevent it maintaining
itself at all in its difficult habitat (cf. also p. 187).

The considerable gap in characters between the other two
prostrate forms and the type would imply that they too are
what we may call '‘all-or-nothing” forms, which may with
some justice be called ecological subspecies.

The distuxction between them and the less specialized ecotypes
would then be that they can only maintain themselves as a
relatively pure population, whereas in cases such as that of
Gregor’s Plantago marttima, each population contains numbers
of ecotypes, none of them very sharply defined, and all inter-
crossmg and connected by intermediates. In this latter case, the
species is polymorphic, the selective balance needed to maintain
the polymorphism (see p. 97) being a balance between the
selective effect of a wide range of habitats. In the former ease,
however, true polymorphism is absent, and a highly adapted
type is niaintained as an ecological subspecies in a special habitat
by means of a considerable degree of isolation.

Tills type of specics-structurc could readily evolve out of the
more usual multiple ecotype structure; doubtless various inter-
mediate conditions will come to light as research proceeds.

The two forms of Ajuga (p, 267) have shown us how the

278 evolution: THE MODERN SYNTHESIS

peculiar conditions of cultivated land may combine with climatic
selection to cause a partly ecological, partly geographical diver-
gence. In other cases, cultivation may induce purely ecotopic
divergence. Thus in some plants, e.g. the weeds Caucalis arvensis
and the fool’s parsley Aethusa cynapium, dwarf strains charac-
teriTC stubble-fields. The taller strains which also regularly appear
earlier in the sea.son in the same fields are eliminated each year
through decapitation of the flower-heads by the reaping machine
before ripening (Salisbury, 1939). It is quite conceivable tliat
good dwarf species may eventually evolve, restricted to the
autumnal stubble habitat.

A point of great importance is that, with ecotopic differen-
tiation, the spatial overlap between related divergents may be
very extensive. Overlap may occur with ecocHmatic divergence:
for instance, the red grouse and the ptarmigan overlap in certain
parts of their range over a zone of up to 300 m. in vertical height.
But such forms as the chifichaff and the willow warbler {Phyllo-
scopus collybita and P. trochilus), for example, though the latter
has a preference for more open situations, overlap extensively
and irregularly over most of thek range; so do the meadow and
tree pipits (p. 289), though to a somewhat smaller extent.

Again, though the beU-heather {Erica cinerea) is adapted to
drier mean conditions than the waxbcll {E. tetralix), the two
are quite commonly found growing together.

It is not uncommon to find a species in the atypical conditions
at the margin of its range adopting a peculiar habitat. Thus the
reed bunting {Emheriza schoenkhlm) in north-west Scotland
and the Isles is found nesting on small islands in lochs, among
low bkdb scrub (personal observation). This is due to the absence
of the marshy willow coverts which it normally frequents, and
to its general preference for low shrubby vegetation near water,
which here is largely confined to islets, since it is elsewhere grazed
down by sheep. The swallow-tail Papilio machaon, normally an
ordinary open-country butterfly, inhabits fens on the edge of its
range, in Britain. In this and other cases, the reason for the changed
habitat is unknown — e.g. for the extension inland in the north-
western regions of Britain, and notably on St. Kilda, of the. rock

279

SPECIATION, ECOLOGICAL ANU GENETIC

pipit, which normally is rigidly confined to the maritime zone
(Nicholson and Fisher, 1940)* As with Sileve (p. 269), the change
of habitat causes an overlap of range with a related species, the
meadow pipit. Gdmmmus duebeni^ mainly brackish water in
Britain, is the chief fresh-water species of Ireland (Beadle and
Cragg, 1940).

The rock pipit is of great interest, since this itself is an eco-
logically differentiated subspecies of Anthus spinoletta, most of
whose numerous subspecies arc styled water pipits, owing to
their preference for streams. All are confined to barren country:
some, like A. s. spimletta^ to alpine areas; others, like A. 5. n//;e 5 -
cens^ to mountains or to low barren areas in the far north. This last-
named subspecies shows a transition towards the habitat-pre-
ferences of the rock pipit (- 4 . 5. petrosus)^ since it is rather frequently
found on steep slopes above sea-cliffs. The combination of
ccotopic and geographical divergence is thus well illustrated by
the forms of this polytypic species.

Sometimes the ecological isolation is concerned primarily with
breeding-places, the forms often mingling while feeding. This
occurs, for instance, with the British Hirundinidae, the swallow
[Hirundo rustica) nesting only under shelter, usually inside build-
ings, the house martin {Delichon urbka) only on houses (or rarely
on cliffs), the sand martin (Riparia riparid^ only in sand-cliffs,
usually along rivers. The house martin, by the way, provides
an example of the effect of human agency in altering range.
This species was originally confined to cliffs as breeding-places,
but its adoption of houses has both changed its type of nesting
habitat and much extended its distribution.

The reverse condition is exemplified by the terns, Sterna^
where several species may show similar nesting habits, but are
differentiated in regard to their feeding. Formosov (in Cause,
1934, p. 19) cites a case where four species nest in a single crowded
colonial breeding territory (though the species tend to keep
separate within the colony), and ail co-operate in driving away
intruders. But their feeding habits arc quite distinct; three fish in
different types of water, and the fourth feeds excusiveiy on land.

The house and tree sparrows {Passer domesticus and P. montams)

280 EVOtUTION: THE MODERN SYNTHESIS

may also be mentioned, die former essentially a parasite or
commensal of man, the latter restricted to open country away
from buildings. However, in certain eastern palearctic regions,
outside the range of the house sparrow, the tree sparrow has
taken the other’s ecological role and constantly associates with
man. This shows how ecological distribution may alter in rela-
tion to the absence of related competitors, just as was shown for
regional distribution in the case of the mountain hare (p. 246).

As mentioned later (p. 322), the small size and rigid instincts
of insects appear to favour ecotopic and ecobiotic differentiation
to a much greater extent than in higher vertebrates.

Before returning to the biological problem raised by the
existence of overlap and the consequent absence of spatial isola-
tion, we will briefly deal with ecobiotic divergence, where the
main adaptation is to a mode of hfe rather than to a habitat.
Here the opportunities for overlap are greatest. One may, for
instance, find half a dozen good species of Geranium or of
Veronica in one hedge or bank, five good species of blue butter-
flies (Lycaenidae) on a single chalk down. All the six common
species of British titmice may frequently exist together in a
single wood, although here they also show ecotopic preferences,
a coniferous wood being quite likely to harbour only die cole
tit {Pams ater), an alder grove by a swamp only the willow tit
{P. atrkapilki}, while the longtailed tit {Aegithahs caudatus)
is somewhat local and tends to frequent rather open bushy
country.

Sometimes size is the decisive factor. The greater and lesser
spotted woodpeckers (Dryobates major md D. minor) are extremely
similar in appearance and habits, but the one is three to four
times the weight of the other. Such differences in size are doubt-
less correlated with difference in the food taken: this holds for
the similar case of the two very similar falcons, the large peregrine
[Falco peregrims) axid the small merlin (F. columbarius). Similar
cxarnples from North America include the common and fish
crows, Corvus brachyrhynchos {corone) and C. ossifragus, the latter
having also well-marked ecological preferences, and the hairy
and downy woodpeckers, Dryofcflfes vilhms and D. pubescens.

28 i

SPECIATION,' ECOLOGICAL. AND GENETIC

Mr. M. A. C. Hinton, tells me that the same holds for shrews
(Soricidae). In Britain we have the common and pigmy species,
Sorex araneus and S. minuius; other size-differentiated sets of
species occur \vithiii ■ the family. In Hemicentetes {p. 287) the
differentiation is by toodi-size, not body-size. Other obvious
examples are to be found in bats, foxes, toads, Mustelidae (stoat
and wczsel, Mustela ermineus andM. nivalis^ ^ cats (e.g, jaguar and
ocelot, Felts onca and F. pardalis) and the curlew^ and whimbrel
(Numenius arquata and N. phaeopus). That the size of predator and
prey is often closely adjusted is shown by the experiments with fish
and water-boatmen mentioned on p. 469. Presumably the size-
difference between pairs of species differentiated on this basis
must reach a certain level before the two types cease to overlap
appreciably in regard to the prey taken, and so gain maximum
advantage by their differentiation. It is relevant that in the
woodpeckers, the ratio of size of the smaller to the larger species
is very similar in Europe and America, as if a certain degree of
size-divergence were necessary to secure the optimum exploita-
tion of the environment by species-pairs of this type. A quanti-
tative comparative study on such size-differences in various
groups should yield interesting results.

With reference to woodpeckers it is interesting that in North
America the group shows a much greater range of ecological
divergence than in the Old World. For instance, acorn-storing,
cat<;:hing insects on the wing, egg-stealing, sap-sucking, etc., arc
characteristic of American species only (see Bent, 1939)* It would
be interesting to try to discover the reason for this and other
similar cases of differential ecological radiation.

Feeding habits arc the commonest source of ecobiotic diver-
gence. For instance, among the British finches, the goldfinch
{Carduelis carduelis) is the only one to prefer thistle-heads, the
bullfinch {Pyrrhula pyrrhula) to prefer fruit-buds: the hawfinch
[Coccothraustes coccothraustes) enjoys berries and green peas, while
the crossbill {Loxia curvirostra) is almost confined to pinc-sccds.
Such dietary specialization is naturally often reflected in struc-
ture: the huge beak of the ha^^ finch and the remarkable crossed
mandibles of the crossbill are obvious examples.

382 evolution: the modesn synthesis

The bir( 3 s of prey afibrd equally good examples. Almost every
British bird of prey is, by its vring-shape, mode of flight, beak,
claws, size and general instincts, specialized for capturing a
distinct type of prey — the peregrine {Falco peregrinus) for large
birds such as ducks and pigeons, the merlin {Falco colutnbarius)
for smaller birds, the buzzard {Buteo buteo) for young rabbits, mice
and other small animals, the kestrel {Falco timunculus) for voles and
insects, the sparrowhawk {Accipiter nisus) for small passerine birds,
the hobby {Falco subbuteo) for the swiftest victims, including even
dragonflies and swallows, the osprey {Pandion haliaetus) for fish.

An interesting case of incipient ecobiotic differentiation is cited
by Noble (1930) in the common Japanese tree-frog, Rhacophorus
schlegeUiy this normally lays its eggs in holes in the banks of
rice-fields, but one form deposits them in frothy masses on
leaves overhanging water. There is a very slight degree of
morphological difference, but the two types appear to be geneti-
cally isolated by their breeding habits, and may be expected to
diverge into good species.

Among plants, differentiation of a clearly ecobiotic nature is
on the whole rarer than among animals, but adaptation to special
modes of pollination, by hive-bees, bumblebees, flies, moths,
butterflies, etc., is a case in point. The different degrees of facul-
tative or obligatory parasitism found in the eyebrights {Euphrasia)
and their relatives provides another example, linking up with
the facts discussed in section 4 of this chapter.

The lampreys {Petrotnyzon) show an unusual type of ecobiotic
divergence. As Hubbs and Trautman (1937) point out the
original mode of life in this group appears to be for the animal
after metamorphosis to develop strong sharp teeth and to feed
in a semi-parasitic fashion on other fishes, usually in the sea.
They grow to a considerable size, and eventually reascend small
streams to spawn and die. Several species of this type are known.

A second set of species, however, entirely cease feeding after
metamorphosis. Their gut becomes functionless, and the teeth
are reduced in size and sharpness and become fragmented.
They live in the smaller streams in this dwarf condition for
under a year, and then spawn and die.

SPBCIATION, ECOLOGICAL .AND GENETIC : ■

The dwarfed non-parasitic but degenerate type is thus an
adaptation to an adult existence in small streams. Different dwarf
species appear to have originated independently from several
parasitic ones.

A very different but equally interesting case is afforded by the
koala (Phascolarctus cinereus). This marsupial exists in several sub-
species. The northern, more tropical one is smaller than the
southern (Bergmann^s rule, p. 211), and is restricted to other
species of eucalyptus. According to Pratt (1937) the leaves of
the eucalyptus species preferred by the more tropical subspecies
are rich in cineol and poor in phellandrene, while the reverse is
true of those eaten by the higher-latitude form. Pratt further
maintains that tliis has an adaptive physiological significance,
cineol tending to keep body temperature down, and phellandrene
keeping it up. If this proves to be correct, we have here a case
of geograpliical subspecies which are strongly differentiated
physiologically and therefore ecologically.

An instructive case of the apparent ecological differentiation
of geographical subspecies comes from lizards (Kramer and
Mertens, 1938^). In Istria, Lacerta muralis occurs in two sub-
specific forms, L. m. muralis and L. m. maculimitris, the range
of the latter confined to the west of the peninsula, and its habitat
almost entirely restricted to the neighbourhood of human habi-
tations where refuse is to be found, while the former, a wide-
ranging form, is found on the east of Istria in woods and thickets
far from houses or villages. However, m. muralis becomes an
associate of man in other areas, but only where the countryside
is naturally very bare, and also highly cultivated — as is the case
with the part of Istria inhabited by m, maculiuentris. Thus the
ecological differentiation of the two forms in Istria is apparent
only, caused by the chmatic and cdapliic peculiarities of the
geographical area of m. maculiventris.

Again, the jirds {Meriones) of the Arabian desert are differen-
tiated into three ecological species “distinguished from each other
by characters of skull and pelage, wliich appears to be closely
correlated in such cases with special habits'’ (Chcesman and
Hinton, 1924). Tw^o arc truly desert forms, one nocturnal, and

284 evolution: THE MODERN SYNTHESIS

therefore with larger atiditory bullae, thicker for, and less paflid
'colour, the other dium.ai, with opposite characteristics, and the third,
less specialized, is ecologically restricted to the cultivated fringe.

Examples' of this sort cotiid be multiplied almost indefinitely.
They show how widespread is the tendency to ecobiotic and
ecotopic diSerentiation — ^in other words to a specialized sharing
out of the environmental habitat and ways of exploiting it
among different related species. At the same time they are a
challenge to biologists, since the method by which such differen-
tiation originates is by no means clear. The chief clues arc the
facts concerning “physiological races” in certain animals and
plants (p. 29s), and the existence of local or sporadic variations
in behaviour in certain animals.

3. OVERtAPPING SPECIES-PAIRS

Numerous puzzling cases are presented by extremely similar
species which overlap over much of their range and yet remain
distinct.

Some of these puzzles, like that of the two European tree-
creepers (p. 34s), we have already shown to receive their solution
in the fact of migration and overlap subsequent to divergence
in isolation. It is probable that other examples, like that of the
marsh and willow tits (p. 270) and of the pied and collared
flycatchers [Muscicapa hypoleuca and M. dbkoUis), arc of the same
nature.

A striking case is that of the two crested larks of North Africa,
C&krida ensiata and G. tbekke, already referred to on p. 215.
These differ only in certain apparently trivial characters, such as
the lengtli of bill, and whether the song is given from the ground
or on the. wing. Their overlap is extensive, but by no , means
complete. ,It ,is quite possible that here, too, .migration .after.,
ccoclimatic differentiation in isolation has been responsible.

Slightly different cases of overlap arise when two waves of
invasion have occurred from different direction at difforent times
(see p. 255). This has been particularly studied in the islands of
die Pacific; thus on the Marquesas, Mayr (1940) finds two

SPECIATION, ECOtOGICAL ANB GENETIC , 285

forms of fmit-<love, which ordinary taxonomic practice would
be inclined to regard as subspecies, living together without any
hybridization. Then we have overlap due to the end members
of a chain, of subspecies_ meeting and behaving as species (p. 244)-
In all, ten or a dozen examples of this, phenomenon arc known
from birds alone. The distribution of the three subspecies of Rana
escutenia is very peculiar (H, W. Parker, in Utteris)^ UsuaHy they
are kept from interbreeding by, differences in breeding-times, but
in certain zones hybridization occurs. Four-fifths of the species of
gall-forming Cynipidae infest the oak (Hogben, 1940). Three
species, including two of Neuroterm^ often occur on the same leaf.
Hogben suggests that agamic reproduction has facilitated tills
divergence.

An interesting case is afforded by the blue butterflies. The
chalk-hill blue {Lycaena corydon) and the Clifton blue (L. thetis)
must be ecologically slightly different in their requirements, since
the latter is found more commonly near the sea, and even
where they overlap, one is usually more abundant than the
other in particular spots. On the other hand, they do overlap to
a considerable extent, and, although their times of emergence
arc not identical, they are frequently to be found flying together.
The browmish females of the two species arc so similar that no
entomologist would undertake to assign a single specimen to
its correct species merely on its appearance. The males, however,
arc strikingly distinct, that of L. thetis being a rich azure, ot
L. corydon a very luminous pale blue.

An example which has been a source of confusion to ornitho-
logists for over a century is that of the genus Tachyeres 01 steamer
ducks, so called from their habit of racing over the surface of
the w^atcr, chuniiiig its surface with their wings like paddle-
wheels. In addition, some can fly; and these have long been
known to be smaller and to have larger wings.

For many years the belief was firmly held by many authoridcs
that 0!ily one species existed, and tliat the flying stage was passed
through in youth. However, Murpliy (1936) has definitely estab-
lished that three distinct species exist— one flightless form from
the west coast of southern South America, a second from the

286 , evolutioh: ;the modern, synthesis

Falkland Islands, and a flying form with a range including that
of both the others and also spreading some distance up the east
coast. The three forms are distinguishable at all stages. The
flightless forms are considerably heavier, especially the mainlmd
type, the males of which are almost double the weight of flying
males. An interesting point (derived from an analysis of Murphy s
da t a , but not noted by him) is that the sexual disparity in size
is much greater in the flightless species. The d/? weight-ratio
in the mainland flightless form is i -47, in the flying form only
I *17. One may Conjecture that this is due to sexual selection in
favour of more powerful males being no longer counteracted
by natural selection in relation to efSciency of flight.

Though the wings of the flightless forms are smaller, the
wing-muscles are normal, since a great amount of energy is
needed for the “steaming”, which is rather faster than in the
flying species. The calls of the three species appear to be diJflerent
and the Falkland flightless species shows genetic tameness.

There is an ecotopic as well as an ecobiotic distinction between
the flightless and flying forms, the latter being found on fresh
water as well as on all regions of the coast, while the flightless
species arc restricted to the coast, and to such regions of it as are
not subject to strong tidal fluctuation.

It would seem clear that both flightless forms have been derived
from the flying form by further specialization of the curious
form of surface locomotion which is already well developed in
it. But whether they have been independently derived or have
differentiated into the mainland and Falkland form subsequent
to losing their powers of flight, is uncertain, as is the mode by
which isolation between flightless and flying forms occurred in
the first place.

The two shesurwaters Puffinus grisms and P. tenuirostris are
closely similar except that the former is larger. In New Zealand,
only P. griseus is found, while P. tenuirostris breeds in southern
Australia. Wood-Jones (1936) considers tliat both species breed
on the same island off Tasmania. This finding would indicate
that die two species have remet after geographical differentiation.

Two close and overlapping species of the insectivore Hemi-

SPECIATION, ECOLOGICAL AND GENETIC 287

centetes seem to be ecobiotically dififerentiated in relation to size
of prey, one having considerably smaller teeth (Butler, 1941).

We now return to the extremely important question of how
related species which overlap spatially are in nature prevented
from intercrossing. It is clear on general grounds that wherever
such crossing is possible, whether it results in fully fertile, partly
fertile, or infertile offspring, or is itself wholly sterile, its occur-
rence will usually be a biological disadvantage. This is obvious
whenever loss of fertihty is involved; on the averse, individuals
whose mating produces no offspring, or ofBpring which them-
selves show partial or complete infertility, will be less fully
represented in later generations than individuals whose matings
are fully fertile. But even when the offspring of two distinct
types arc fully fertile, their production may be a disadvantage.
This will be so when the parent types are ecologically well
adapted to distinct environments or modes of life: for ex hypothesi
their hybrid products will be less well adapted.

Accordingly we may expect natural selection to operate to
prevent the crossing of related but distinct forms under the
following conditions: (i) when the two forms overlap spatially
and consequently have the opportunity of interbreeding; and
(2) either, (a) when divergence has proceeded far enough for
crossing to be attended with reduction in fertility, of whatever
nature; or (b) when the two forms arc subject to strong selection
adapting them to distinct environments or modes of life.

We shall, on the contrary, not expect such special barriers to
mating to be erected when (i) the two forms do not overlap
spatially; or (2) they overlap spatially but arc capable of pro-
ducing fully fertile offspring, and are further not subject to
strong selection promoting adaptive ecological divergence.

Let us see how these deductions work out in practice. The
simplest method by which related forms can be prevented from
crossing is by the possession of distinct breeding seasons, and this
is frequently found to occur. An inspection of the breeding
seasons of the marine animals of the Gulf of Naples shows many
examples, and a number of cases arc known among flowering
plants, moths, etc.

288 EVOIUTION: the modern SyNTHESIS

Tliis, however, is not always practicable. In temperate Mci
arctic climates, it is inevitable that the bulk of the bird species,
for instance, will breed at almost the same period, and the same
will often apply to flowering plants. With flowering plants,
various alternatives arc possible. Either the flowers may open
at different times of day; or a sharp distinction in colour or form
of flower may be evolved, which, since bees tend to visit a
number of similar flowers in series, will reduce the chance of
cross-pollination; or the two forms may become adapted to
pollination by different species of insects. Factors making for
reduced fertility of foreign pollen may also be encouraged.

"W^ith higher animals, the most obvious method will be the
encouragement of specific mating reactions. Numerous inter-
esting cases of this exist. luJOfosophila, no example is known where
different species will mate as readily as do individuals of the same
species. Races A and B of D. pscudoohsctif a almost certainly differ
in the males’ stimulating scent. The sprosser and the nightingale
will mate in captivity, but do not do so in nature (pp. 246, 254).

In general, it w'Ol be found that among birds and other higher
animals overlapping and related forms frequently differ markedly
in regard to some character connected with recognition. These
recognitional characters may be auditory, visual or olfactory,
and they may be common to the group as a whole, or confined
to one sex. In any case, they often have a function in relation to
keeping the group defined and preventing interbreeding with
other groups, though they may and normally will have other
functions as well. For instance, in gregarious animals, recog-
nitional characters common to aU individuals may serve to keep
the group together on migration or to give warning on occasions
of danger, or may enable the young to recognize their parents
or others of the species, or prevent mating with members of
closcly-rclatcd species. Recognition cliaracters confined to one
sex, in addition to facilitating recognition by offspruig, may
serve for recognition between the sexes, or between members
of the same sex, or both (c.g. the “moustache” of the male
flicker, Cohptes auratus. Noble and Vogt, 1935; and see dis-
cussion in Huxley, 1938c); or have some sexually selective func-

SPECIATIdN, ECOLOGICAL AND GENETIC 289

tioiG either in regard to choice of mate, as in mfFor blackccck,;
or in, promoting readiness to copulate, as apparently with most
monogamous birds. In all such events, however, any marked
difference between the recognition characters of related forms
will have a fuiiction which we may call that of group Jisiimiive-
ness, in that it will promote the unity of the group wiiicli it
characterizes and give it a sharper biological delimitation from
Other related groups (see also p. 545).

Lorenz (1935) has emphasized the biological value of distinc-
tiveness as S!Kh in all characters serving to elicit , behaviour-
reactions in other individuals {allaesthctic characters: Huxley,
19384 ; but he has not emphasized tliis frequently superadded
function of group distinctiveness, which is of biological value
only in so far as it keeps groups apart: sec also Lack (1941).^

As examples of distinctive characters serving as barriers against
intercrossing, we may note the fact that bird species that are
closely similar in appearance and overlap spatially frequently
differ strikingly in their calls and songs. This is best exemplified
in the songs of the three British species of Phylloscopus, the
chifFchaff [Phylloscopus coUyhita), willow warbler (P. trochilus),
and wood warbler {P, sibilatrix), notably the first two. It was
by their songs that Gilbert White in 1768 was able to be the
first to distinguish all three. Other good British examples are
the meadow and tree pipits, and die marsh and willow tits
(p, 270). As Mayr (1940I7) writes of certaiji almost indistinguish-
able overlapping species of minivet {Perimeotm), Cisticola, etc.,
‘‘the birds themselves arc apparently not deceived, diough the
taxofiomists are’', since hybridization seems not to occur.

Song in these forms thus has a dual function. Since its primary
function is to advertise the possession of territory, it must be
striking; but since a secondary function is to advertise the fact
only to members of the same group, the song of related and
overlapping forms must be markedly different. It is both dis-
tinctive per se and also group-distinctive.

* Molony (1937) in a recent book has done useful service by drawing
tion, fnnii the standpoint of ficld-naturalist, to tiie group function of recognitkni
iiiarks, especially in keeping the young in the group and within the group
tradition. . ,

290 EVOitJTION: THE MODERN SYNTHESIS

Heinroth and Heinroth. (i 924 ~d; P- 4-9) b^ve a general

discussion of the songs of the thrashes (Turdus), which illus-
trates the same principle.

it is interesting to note that the marked differentiation of the
various species in song bears no obvious relationship to their
equafly striking differentiation in visual appearance. In visual
appearance, the original thrush type (brown with spotted under-
parts) has been modified in various species (primarily in the
males) by the addition of striking characters like the black head
and chestnut breast of the American robin {Turdus migratorius)
or by a total transformation as in the blackbird {T. merula)
or the ring ouzel (T. tor^uotus'^. This striking differentiation in
visual appearance due to the need for specific distinctiveness is
seen in many other birds, either in the males only or in both
sexes. Obvious examples include that between the whinchat and
stonechat {Saxicolu rubetrei and S. torguatd)', the special colora-
tions of the goldfinch {Carduelis carduelis) or the redpolls (C.
jiammea) compared with other members of the genus; the white
nape-patch of the cole tit {Parus ater) as against its absence in the
marsh and willow tits; the colour of the crest in goldcrest and
firecrest {Regulus regulus and R. ignicapillus) respectively; the
general colour of the blackbird {Turdus merula), as against the
more typical brown and spotted song-thrush (T. philomelus).

SPECIATION, ECOLOGICAL AMD GBMETIC ; :

crossing is foiind iii die dart oi spkulum' amoris in the o¥erlappiiig
snail specks ^^Cepaea Iwriensis and Cepaea nemomlis. That of
nemaralis is bigger and more powerfully ejected than, that of the
other; as a result, if members of the two spedes attempt to
pair, the weaker fails to provide adequate sexual stimiiliis, wliilc
that of the other is. so powerful that it causes the weaker to
shrink away (Diver, 1940, p. 326).

In the deermicc Peromyscus, several cases occur of distinct sub-
species -sharing the same geographical area but being adapted to
different habitats. Thus in P. mankuktus {Dicc^ 1931) members
of short-tailed prairic-dwciling subspecies may overlap witlimcni-
bers of various long-tailed woodland-dwelling forms. Speci-
mens of P. m. osgoodi and P. m. artemisii may even be caught in
the same traps. We may conjecture that in this last case tlic
discontinuity is here preserved by some difference iii m,ati,ng
reactions. The same appears to be true for the two good species
P. leucoptiSy the wood-mouse, and P. gassy pirns, the cotton-
mouse, the former northern (and western), the latter 'southern.
(Dice, I940fi). Though interfcrtile in captivity, the two remain
perfectly distinct in the small area where their ranges overlap.
Dice considers that this psycliosexual type of isolation is the major
one in Peromysms; ecological isolation may occasionally reinforce
sexual, -but tends to break down at the margins of habitat zones,
w-'hcrc the ecological relations arc somewhat abnormal. Tliis is
doubtless not general, though Spencer (1940) regards psycho-
sexual and reproductive barriers as primary in the^ differentiation
of Drosophila, In the gartcr-snakc, vrdinoidcs, eco-

logical differentiation, largely related to terrestrial or aquatic
habit, may be primary (Fitch, 1940); aquatic habit, which arises
in arid areas, permits much larger size; different ecological
subspecies frequently come to overlap geographically.

Where neither psycliosexual nor ecological isolation is (>pcr
tive, two (or more) subspecies w'’hich meet owing to range-
changes may fuse into one. Botli Dice and SuniDcr, the two chief
authorities on the biotaxonomy of the genus ftrowysms, consider
thatsuchpolypliylcricsubspccicsarcnotinfrcqucntinit (seep. 248).

Besides such sexual characters wliich act as barriers to inter-

evolution: TlMi MODERN synthesis

_ ' ^ assortativc mating, non-sexual recognition
have^a simUar effect by keeping members of a
■ '-bars of various related
or the striking and diversely coloured
■ i of ducks will serve as
innumerable cases of group-

crossing by promoting
characters may I— .-
species together. The distinctive wmg-
birds (e.g. sandpipers, etc.)
specula on the wings of different species

visual examples, while there are i..

distinctive but non-sexual call-notes.

Olfactory characters, of course, wdl play a par
Uke mammals and insects where the sense of sn
important than it is in birds. It is probable that sue
are group-distinctive in Drosophila.

It must be admitted that there are many cases whci
satisfactory explanation of the absence of intercross
overlapping and related species is as yet forthcoi
{1940) has enumerated some of these; for instaiKC,
numerous and often scarcely distinguishable species
moths of the genus Cratttbus, many of which may
the same ground. We may presume that slight bi
olfactory stimuli control the mating-reactions, but 1
hypothetical; and the problem of initial divergence
also A. P. Blair (1941) on overlapping species oftre(

SPECIATION, ECOLOGICAL AND GENETIC 293

their geographical range so as to overlap, intercrossing may occur
freely, and the oiFspring be fertile.

We have already mentioned examples of this, notably in the
flickers in America, the crows and other birds in the Old World,
and various plant species (p. 115).

Another excellent case concerns the red grouse (Lagopus
scoticus) of Britain and the willow grouse (L. lagopm) of Scandi-
navia. These, though generally recognized as “good species”, arc
closely aUied, and their considerable differences arc clearly die
result of geographical divergence. However, there has been no
pressure of selection operating to erect barriers to inter-crossing,
and accordingly, when either species is introduced into the
geographical range of the other, the aliens, contrary to the ex-
pectation of the sportsmen responsible for the introduction, have
not maintained themselves, but have quickly become incorporated
into the indigenous species. A similar lack of barriers may exist
between wholly unrelated moths. Mr. Ford tells me he has found
male burnets [Zygoma filipendula) attracted by female oak-
eggars [Botnbyx qiiercus). Here the waste arising from the actual
production of hybrids is absent, and ecological preference normally
isolates the two forms.

Dobzhansky (1937, p. 258, and, more emphatically, 1940) is
of the opinion that restriction on interfertihty (in the broadest
sense) will be brought about only by selection, whereas most
authors believe that random accumulation of differences in stocks
isolated from each other will in the long run inevitably lead to
some restriction, whether by reducing the frequency of unlike
matings or the fertility or viability of their offspring (p. 371).
There is no question that, even if an initial reduction of inter-
fertility may be due to the accidents of divergence, it may subse-
quently be increased by selection (p. 360). In this connection it is
worth noting that in the case of the crows (p. 248) and the grackles
(p. 251), the zone of intergradation is narrowest where the
two forms are presumed to have been longest in contact, and
where therefore selection aimed at the reduction of biological
waste will have had more chance to exert its effects. It wopld
be of the greatest interest to test individuals from the most

294 ' EVOIXJTXON:- THE MODERN SYNTHESIS ■

recent and the longest-estabKshed areas of contact to see whether
they are in point of fact generally different in their psychosexual
reactions or in the viability or fertility of their offspring.

★ * * * *

The problem is not simple. We must remember that related
species now found together in one region may have differentiated
in quite separate regions and have been brought together later
by migration. The British Isles, for instance, contain represen-
tatives of three or four regional faunas — ^Northern, Central
European, South-eastern European, and Lusitanian. Only after
the end of tbe Ice Age were these brought together in our islands,
so that the ordinal differentiation of many forms, such as the
common and mountain hare (p. 246) or the carrion and hoodie
crows (p. 248) occurred in different regions.

There is also plenty of evidence to show that, as we should
expect, character-divergence shows a correlation with genetic
intersterility. In the deermice of the genus Peromyscus, Dice
(1933b) has shown that whereas subspecies are mutually highly
interfertile, and “good” species belonging to the same species-
group, as ^fined by morphological resemblance, are moderately
so, those belonging to different species-groups are wholly inter-
sterile. But the condition appears to differ for different types of
animal. In deermice marked intersterility appears with a small
degree of morphological difference (sub-generic or “sub-sub-
generic”), hi pheasants, so far as the evidence goes, it begins
with generic difference, and in some ducks at least, even generic
CTOsses fnay be quite fertile. There is also evidence that different
types of Canidae may be interfertile in spite of wide taxonomic
divergence. In any case this type of sterility is often associated
with readiness to mate, and if so is quite diferent from the
special barriers to intercrossing we have been considering, which
usually operate to prevent mating rather than to reduce fertility.

It is chax that many more facts are needed, based on the
analysis of a large number of crosses between related species of
ecological and geographical type. None the less, the following
deductions appear to hold. First, that with the same degree of

SiM'iCl ATiON, HCOKxaCAL AND CBNHTIC 295

general charactcr-<livcrgcncc (excluding characters promoting
assortativc mating and acting as barriers to interspecific crossing),
types which have diverged in geographical isolation will show
less cfFcctivc barriers, direct or indirect, to fertility, than those
which show ecological divergence iti the same area. But secondly,
that as regards characters promoting assortativc and impeding
interspecific mating, the ecological type of spcciation within the
same region will accentuate the degree of character-divergence,
largely by promoting the evolution of characters accentuating
group-distinctiveness (p, 289),

This last point is clearly valid, since such characters arc only
serviceable if they arc immediately recognizable, or at least
sharply and qualitatively distinct. It is further likely that ecological
but spatially-overlapping difFcrcntiation will promote a more
rapid and thorough divergence in general characters, since more
complete adaptation to the two ecological niches will be a!)
advantage to both species. And for this reason it will be difficult
to attach precise meaning to comparisons as to degree of diver-
gence between the ecological and the geographical types of
spcciation. What we can say, however, is that when the degree
of general character-divergence between two overlapping species
is slight, and we yet find considerable barriers to intercrossing,
we shall expect to find lower barriers between two non-over-
lapping (geographically isolated) species with the same degree
of character-divergence. And this, so far as we can judge on the
evidence at present available, is true.

4. BIOLOGICAL DIFFERENTIATION

A special type of ccobiotic divergence, and one which from its
practical bearings has recently received a great deal of attention,
is that usually known as biological (or physiological) differen-
tiation. By this is meant the divergent adaptation of separate
groups of parasites or phytophagous ammals to particular hosts
or food-plants. Reviews of the subject have been given by
Thorpe (1930, 1940), from which we select most of our example3.
(References given only for cases not cited from Thorpe.)

296 evolution: THE MODERN SYNTHESIS

Hie most striking points about this kind of difierentiation are
as follows; First, all gradations are found from incipient physio-
logical subspecies to full species characteri2ed by complete inter-
sterility and morphological differences. Secondly, however, the
visible morphological divergence lags further behind the invisible
physiological (including the reproductive) than in any other type
of differentiation. Thirdly, barriers to intercrossing, largely it
would appear on a psychological basis, appear to be speedily
set up by selection between Ae diverging groups. And finally,
Ae biological differentiation in its early stages appears usually
to depend upon an interesting form of organic selection (the
Baldwin and Lloyd Morgan principle; p. 304) operating in its
niodificational phase through olfactory conditioning.

Let us now examine some well-analysed cases in Ae light of
Aese principles. The maggot of Ae Dipteran known as Ae
apple fly {Rhagoletis pomonella) is very destructive to certain
fruits. It appears originally to have been a parasite of a species
of hawAom (Crataegus) in NorA America, but this genus is
now but rarely attacked. It exists in two main forms, differing
in no visible characteristic except size, but confined to different
host-plants, Ae larger attacking apples and related fruits, Ae
smaller blueberries and huckleberries. The difference in size
averages about 30 per cent, and Acre is no overlap. In some
states, e.g. Maine, Ae “blueberry maggot” has been immemorially
established, while Ae date of introduction of Ae “apple maggot”
and Ae course of its subsequent spread are known. It is extremely
hard to raise one form on Ae host-plant of Ae oAer. Crosses
can be obtained artificially between Ae two forms, but only
wiA difficulty, Aough Ae offspring are viable. There seems no
question that Ae two forms are, reproductively speaking, good
species, in spite of Aeir morphological similarity. The original
Afffrentiation may have been into hawAom and blueberry
forms, Ae hawAom type later becoming adapted to apples, but
this is uncertain.

The two forms of Ae Homopteran Psylla mali provide a very
similar case. Here again Ac adults of Ae two types differ morpho-
logically only in size, but arc exclusively confined to apple and

297

SPECIATION, ECOr.OGICAI. AND (JENliTIC

liawtlioni respectively. However, the difFerciitiatioii has gone a
little further than in Rhagolctis, (or it lias so far proved inipo.ssiblc
to make the one race lay eggs on the other’s food-plant, or t«i
obtain cross-breeding. In addition, slight morphological differ-
ences have developed between the two types in the nymphal
stage. An interesting point is that the hawthorn race is parasitized
by certain Chalcids and Proctotrypids, but the apple race is not.
Such forms, though for museum purposes it is convenient to
leave them in the category of “biological races”, must be con-
sidered by the evolutionary biologist as distinct species.

A much smaller degree of divergence is shown by the bio-
logical races of the ermine moth Hyponotneuta padella, one being
adapted to apples, the other to hawthorn and blackthorn. No
structural features separate the two, although there arc slight
colour differences; the colour of the forewings in the species as
a whole ranges from dark grey to pure wliite, and the dark
forms are more abundant on hawtlioni, the white form on
apple. The apple form is usually a leaf-miner in its first larval
instar, the other not; the pupae of the apple race arc usually to
be foimd in neat packets or rows, with a dense cocoon, while
those of the hawthorn and blackthom race are generally scattered,
and the cocoon is very flimsy.

If the moths are given the choice of food-plants, they show
a decided preference for their normal host (8o per cent in the
ease of the apple race, 90 per cent in that of the hawthorn one).
Although the hawthorn and blackthom “sub-races” are indis-
tinguishable in most ways, they arc actually separable on the
basis of egg-laying preferences (80 per cent and 70 per cent for
tire hawthorn and blackthorn forms respectively).

The food-preferences of the larvae also, though marked, are
not fixed; they can be induced by starvation to feed on the
“wrong” food, though the resultant imagines arc generally
undersized and often infertile. Finally, the mating-preferences
are only relative. Elaborate and large-scale experiments showed
that the attraction between individuals of the same race was
about twice as strong as tliat between diosc of different races.

Owing to these various preferences, the different races must

298 evolution: the modern synthesis

keep themselves fairly distinct in nature, although occasional
crossing probably occurs. The races are thus “biological sub-
species”, and deserve trinomial recognition.

Some remarkable results have been obtained on crickets.
Thus in Oregon the snowy tree-cricket Oecanthus nivalis exists
m two forms. The one race is a tree-dweller, and lays its eggs
singly on the bark of apples and similar trees; the other lives
in bushes, and lays its eggs in dense rows inside the pith of rasp-
berries and si m ilar shrubs. The two types show no visible differ-
ences, but are immediately distinguishable by ear, the shrub
race uttering its repeated notes with a frequency only about half
that characteristic of the other. The form foimd in the eastern
United States resembles the tree race in its habits, but is charac-
terized by a distinctive song of its own. It is always difficult
and sometimes impossible to make the shrub form lay its eggs
on trees, and vice versa.

Another cricket, Netnobius fasciatus, is in Iowa divided into
several races reachly distinguishable by song-frequency. Again,
each race has its own ecological niche, but shows no or negligible
morphological or colour differentiation. In various regions, the
different races are found side by side. In this case crosses have
been made between members of distinct races, and the results
indicate that the song-difference is genetically determined, and is
dependent on several interacting genes.

Since the song of crickets is an epigamic character, and since
recent work (cf. Pumphrey and Rawdon-Smith, 1936) indicates
that insects must rely specially upon differences in song-frequency
(rather than pitch) for auditory discrimination, this seems a clear
case of the evolution of a special barrier against intercrossing
between ecologically-differentiated groups (pp. 287, 385). The
different types are perhaps best regarded as well-marked sub-
species, though well on the way to complete independence.

In wood-boring beetles of the family Gerambycidae, results
were obtained very similar to those in Hyponomeuta. Yaxious
species are differentiated into “biological subspecies”, normally
confined to one or a few kinds of wood. In every case (e.g. the
hickory and wild grape strains of Cyllene pictus) the larvae can

SPECIATION, BCOLOCiCAL AND GENBTU: 399

be made to^ abandon their preferences and to live on and even-'
tnally even to prefer a different type of wood; but the difficulty
of inducing altered habits, and the initial mortality, differed very
greatly in different species. In one case at least moderate assortativc
mating preferences were exhibited.

In Hawaii, in the beetle Plagithysmus, the process of biological
differentiation appears to have proceeded further, to the stage
of good species. Three species, each confined to its own food-
plant, and apparently never crossing, may occur together within
the space of a few yards. Distinct morphological differences
between them exist, but arc so slight that one entomologist has
written, ‘It is hardly conceivable that species can be more closely
allied than these and yet remain distinct/’ Such a judgment
reflects the natural preoccupation of the taxonomist with visible
structural diagnostics: we. now know that groups may remain
perfectly distinct though morphologically indistinguishable.

Many forms of gall-producing insects (e.g. Cynips) are distin-
guishable solely or mainly by the type of gall to which they give
rise.Tliesc will probably turn out to be adaptive “biological races”.

Sometimes these biologically adapted forms arc also geo-
grapliically separated. Thorpe (1930) mentions the following
illustration. In die Orient, the red scale ChrysGinphalus aurantii
is parasitized by a chalcid wasp, Camperklla bifasciata. But when
this was introduced into California to cope with the scale pest
there, it was found to be useless. Although C. aurmtii in Cali-
fornia is indistinguishable from C. aurantii in the Orient, it must
be different physiologically, for though the chalcid parasite lays
its eggs on it, the larvae are always destroyed by phagocytosis,
instead of developing freely at its expense, as in the Orient.

It is interesting that the concept of biological races in wood-
boring and phytophagous insects was advanced sixty-five years
ago by W but that his conclusions remained virtually un-
noticed until 1933.

Biological races of diis type are not eonfmed to insects, but
are found also in many other groups, such as Arachn.ida, Ncma-
toda, Protozoa, Bacteria, Fungi, and some liighcr plants. Among
die latter, the different races of mistletoe may be mentioned.

300 EVOLUTION : THE MODERN SYNTHESIS

each characterized by ability to parasitize a particular host (p. 308).

In Arachnida, the evidence, though not fully conclusive, makes
it very probable that the mange-mites of the species Surcoptes
scaber are split up into biological races each adapted to a par-
ticular host — goats, sheep, camels, dogs, horses, guinea-pigs,
rabbits, men, etc.

In the free-living mites of the genus Paratetranychus, the process
has continued to full speciation, P. pilosus attacking only fruits
of deciduous trees, and P. citri only citrus frtiits. Though cross-
mating occurs readily, it is never fertile; there are very slight
morphological differences (so slight that the two forms were
long regarded as belonging to a single species), and also slight
difierences in habits, egg-kying and food preferences, and in
distribution.

The “red spider” {Tetranychus opuntiae) introduced into Aus-
tralia to combat the spread of prickly pear [Opmtia) is morpho-
logically identical with the “red spider” of orchards and gardens,
but is entirely confined to Opmtia, and appears always to starve
to death on any other food-plant. Here we have complete bio-
logical separation, but no visible divergence.

Among Nematoda, Tylenchus dipsaci appears to be well-
diferentiated into biological races, e.g. jhe strawberry race and
the narcissus race. In Heterodera radkola, on the other hand, the
various biological races can be made to adapt themselves to new
hosts without much diflSculty.

Among Protozoa, the trypanosomes show well-marked “bio-
logical races” adapted to dtferent hosts (see e.g. Duke, 1921),
but in these uniceEular forms it is not certain to what extent
the phenomenon is due to Dauermodijikationm induced by the
difierent conditions.

In Myxosporidia, simikr physiological races also exist (Fan-
thani, Porter and Richardson, 1939).

The phenomenon of biological differentiation seems to be of
common occurrence in certain fungi. The few examples which
follow are taken from the summary by Ramsbottom (1940).
The changes known to mycologists as saltation seem probably
to be akin to the so-called “mutations”. of bacteria (p. 131), and

SPECIATION, ECOLOGICAL AND GENETIC 301

in some cases at least to the Dauermodifikationen of Protozoa.
In any case, in pathogenic forms they often result, as in bacteria,
in changes of virulence towards particular host-strains. In the
rusts (Uredineae), what appear to be true biological races are
widespread. Sometimes the parasitism is so strict that host-species
may be identified by its means, as when, in the difficult group
of willows, one species was identified by its reaction to the rust
Melampsora ribesii-purpurea. Similarly some rusts occurring on
separate but related hosts, and with specific hfe-historics, often
show close morphological resemblances. In such cases we are
clearly dealing with biological differentiation which has passed
the species-level.

The Puccinia of grasses show the same phenomenon at the
level of “biological races”, but pushed to an extraordinary degree
of diversification. Thus from one originally recognized “species”
P. graminis, a second, P. phlei-pratensis, parasitizing Phleutn, was
divided off some thirty years ago. The restricted P. graminis, it
was then found, could be divided into six “forms”, according
as the host-plant was wheat, oats, rye, or various grasses. Each
of these has now been shown to consist of numerous minor
biological races, varying in regard to their infective specificity
for various strains of the host-plant; thus some seventy “physio-
logic forms” have already been detected within the main wheat
form. Ramsbottom considers that die “species” P. graminis
includes at least a thousand separate biological strains, each con-
serving its physiological peculiarities witli “remarkable con-
stancy” (see also below, p. 308).

It is of great biological interest to find that this veritable army
of biological races, which in one phase of its life-history is speci-
fically adapted to several genera and a great many full species
of grasses and cereals, is restricted during its other phase to
quite a few species of the two genera , Berberis and Mahonia.
Two strains which cannot live on the same grass can an 4 do
hvc on the same barberry. This unequal speciaUzation is doubtless
due to the imcqual taxonomic differentiation of the two types
of host-plant, and must assuredly have been accentuated by the
artificial production by man of new strains for the grass phase

302 evolution: THE MODERN SYNTHESIS

to invade. It is an excellent example of adaptation localized in
time to a particular part of the life-history (cf. p. 424).

Of biological differentiation in bacteria we shall not speak,
since it is not certain, owing to the absence of sexual reproduction
in members of this group and their consequent different type
of evolution, whether it really represents the same phenomenon
as in higher organisms (p. 131). This at least can be said— that
strains differing in virulence and in various important biological
and biochemical properties do exist within types that appear homo-
geneous by ordinary criteria, and that the phenomenon in bacteria
and higher organisms must rest on a common and fundamental
capacity for physiological adaptation of strains within a group.

It remains to discuss the evolutionary origin of biological
differentiation in animals. For a considerable time it was. sup-
posed that this was a lamarckian phenomenon, and various
experiments apparently supporting tins view were adduced. To
take but one instance,}. W. H. Harrison (1927) studied a sawfly,
Pontania salicis, whose larvae produce galls on willows. This
“species” contains a number of distinct biological races, each
normally confined to a particular species of willow, and each with
specific egg-laying preferences. However, he was able to convert
the biological race normally confined to Salix andersoniana. into
one adapted to S. ruhra, by restricting specimens for four years to
plants of the latter species. The experiment was continued for
three further years, during which a choice of both species of
willov/ was provided, but the strain remained true to its new
host. It is to be noted that the mortality in the first generation
was very high, and that only gradually was a race established
which could be said to be adapted to S. rubra.

Thorpe cites numerous similar cases, but this appears to be
the most thorough of what we may call the preliminary researches
on this point. Lately, however, Thorpe himself has carried out
beautiful experiments which demonstrate that the lamarckian
interpretation is neither necessary nor tenable. He first of all
(Thorpe and Jones, 1937) reared the ichneumonid Nemeritis
canescetu, which normally parasitizes only the larvae of the meal-
moth Ephestia kuhniella, on those of the wax-moth Acliroia

SPECIATiON, ECOLOGICAL AND GENETIC 3O3

grtsella. This resulted in a significant change in the responses
of the adult females. All female imagines of the species possess
a genetically«determined response to the smell of Ephestia; but
those which have been reared as larvae on Achroia, or have been
brought into close contact with it immediately after emergence,
show in addition an attraction to Achroia which those from
normal hosts altogether lack. Later work showed that this result
of larval conditioning depends on a general tendency to be
attracted by any olfactory stimulus characteristic of a favourable
environment (Thorpe, 1938).

Further work with Drosophila melanogaster (Thorpe, 1939) has
extended these results and shown their general applicability to
non-parasitic as well as parasitic insects. Whereas adult fruit-
flies are normally repelled by the smell of peppermitit, those
which have been reared on a synthetic food medium to which
peppermint essence has been added, are markedly attracted by
the smell of peppermint when tested in an olfactometer. Further,
this response is not abolished (though it is somewhat reduced)
by washing the fully-fed larvae or newly-formed puparia free
of all traces of the medium and of the peppermint essence, thus
provmg that influences operative only during the larval phase
can influence adult behaviour. If not reinforced, the influence
gradually disappears and becomes extinct after about a week.
Filially, in Drosophila as in Nemcritis, it was found that exposure
of the adult insects only, immediately after emergence, to the
smell of peppermint brings about positive conditioning even if
the smell is not associated with any favourable aspect of the
environment^ — the mere fact of the occurrence of the stimulus
at tliis time brings about subsequent attraction to media con-
taining the same substance.”^

To use Thorpe’s own words, ‘"the theoretical importance of
such a conditioning effect is that it will tend to split a population

* As Thorpe (1939) suggests, these results may also explain the interesting
results obtained by Sladdcn and Hewer (1938) on the food-preferena's of stick
insects, for which, prior to Thorpe's work, a lamarckian interpretation seemed
almost inevitable (sec p. 459). It will be of the greater interest to test Sladdcn's
results in the light of Thorpe’s methods, and with a species capable of sexual
reproduction.

304 evolution: the modekn synthesis

into groups attached to a particular host or food-plant, and
thus will of itself tend to prevent cross-breeding. It will, in
other words, provide a non-hereditary barrier which may serve
as the first stage in evolutionary divergence”. We have hero a
beautiful case of the principle of organic selection (p. 523), as
enunciated by Baldwin (1896, 1902) and Lloyd Morgan (1900),
according to which modifications repeated for a number of
generations may serve as die first step in evolutionary change, not
by becoming impressed upon the germ-plasm, but by holding
the strain in an environment where mutations tending in the same
direction will be selected and incorporated into the constitution.
The process simulates lamarckism but actually consists in the
replacement of modifications by mutations (see also Osbom, 1897).

That such a replacement has acmally occurred in the formation
of biological races in insects is strongly indicated by the high
mortahty that, in Harrison’s experiments with Pontunia and many
other cases, often attends transference to a new host. Harrison was
able to transfer his sawflies to a new host-plant by means of their
olfactory conditioning mechanism, but only at the expense of
eliminating those that were genetically best adapted to the old host.
Had previous genetic adaptation gone further, olfactory condition-
ing, while it might still have induced oviposition on the strange
host, could not have given rise to a viable strain upon it.

Once genetic adaptation to a particular host has begun, selec-
tion will step in to prevent the biological waste which would
be caused by the desposition of eggs on other hosts. The mechan-
ism of olfactory conditioning provides a certain reserve of
plasticity; but this plasticity will become hedged about by
genetic safeguards. Genetically-determined attractions to die
normal host will become cstabHshed, and also spedfic assortativc
mating reactions to prevent cross-mating. Thus ecobiotic isola-
tion here has the same general effects as geographical or ecotopic
isolation, but operates by rather a different mechanism, and
follows a somewhat different course as regards the degree of
divergence in morphological, physiological, and reproductive
characters respectively.

Organic selection, but of a quite different type, appears prob-

SPECIATION, ECOLOGICAL ANO GENETIC 305

ably to be operative in lice {Pediculus; summary in Thorpe,
1930). As is now well known, though the human body-louse
and head-louse are so distinct morphologically that they have
received different names, yet head-Hce can be transformed into
the body-louse type by being kept on the body for four genera-
tions. Unfortunately no data exist as to the initial mortality,
though the change seems to have been readily effected. The
two types also exhibit biological differences. Head-hce feed more
frequently but take smaller meals, are more active at lower
temperatures, climb more actively, and exhibit egg-laying pre-
ferences for hair as against cloth. The two types must be con-
stantly exchanging members by migration. It would seem that
we are here witnessing the incipient phase of a process of organic
selection, in which most of the quite well-marked differences
between the two forms still depend on modification. However,
anything which intensifies selection for closer adaptation would,
we may prophesy, speedily bring about genetic and reproductive
divergence. Nuttall suggests that, if man becomes progressively
more hairless, body-lice alone ■will survive. If so, many of their
adaptive peculiarities should become genetically fixed by selection
(but see also Buxton, 1940; Parasitol: 32 : 303).

Organic selection may also operate in song-birds. Some basis
for song is certainly fixed genetically in aU birds, and in some
species this is the whole story. In others, however, there is con-
siderable plasticity, and much of the song is leamt by the yoimg
birds from their parents or other adults. Thus Baltimore orioles
{Icterus galbula) reared in isolation developed a song totally
unlike the normal, and retained it throughout their lives. Other
Baltimore orioles reared with them learned this song and sang
it exclusively, even after their foster-parents’ death (W. E. Scott,
1 90 1-2). By isolating young canaries and allowing them to hear
only the song of the nightingale (on gramophone records) it
has been possible to produce a strain -with a song intermediate
between the canary' and nightingale type.*

Numerous data on the subject are scattered through the

* A brief reference to this experiment is made in J. Om. 75 : 248 (1927)-
Dr. E. Mayr tells me that it was carried out by a fancier named Reich, but that
complete proof was not supplied.

306 evolution: the modern synthesis

Heinroths’ monumental work (1924-6); see also O. Heinroth
(1924), Stadler (1929). In the blackbird (Turdus tneruh), the
chifFchafF {Phylloscopus collybita), the grasshopper warbler (Locus’-
tella naevia) and the short-toed treecreeper {Certhia brachydactyla)
song is innate, and is quite normal even in males reared without
hearing others of their own species sing, whether in isolation or
exposed to the songs of other species.

On the other hand, the whitetliroat (Sybia communis), the tree
and meadow pipits (Anthus trividis and A pratensis), the green-
finch (Chloris chloris), and the chaffinch (Fringilla coelehs) hzYQ to
ieam their songs. Young males if kept in isolation produce a quite
abnormal song, e.g. those of the untaught tree pipit and meadow
pipit resemble the natural songs of grasshopper warbler and serin
finch [Serituis canarius) respectively, while that of the untaught
chaffinch is not unlike that of a lesser whitethroat (Syhia curruca) ^

It is not easy to discover on how many cases the Heinroths*
conclusions are based, and possibly the reality is not quite so
clear-cut. However, the general conclusion that some species
have to learn their song seems inescapable.

Forms in which song is not innate will, if kept with other
species, learn from them. Thus a whitethroat and a linnet (Car-
duelis cannahina) reared together both had an identical song,
resembling a mixture of a robin’s and a skylark’s.

There seems to be a predisposition to learn the normal song;
thus a nightingale which mimicked the songs of various species
with which it had been reared, very rapidly learnt its normal
song on hearing it next year.

Other species if kept isolated will produce an imperfect version
of the normal song, and will learn more or less thoroughly from
other species. Thus the untaught yellowhammcr (Emberiza
citrinella) never develops the complete natural “phrasing’’: one
kept witli a normal linnet developed a song extremely like a
hnnet’s! The robin (Erithacus rubemla) md hhekesip
atricapilla) fall into this category. The song of the song-thrush
(Turdus ericetorum) is almost wholly innate, but can be slightly
modified by “learning”; the skylark (Alauda aruensis) has a song
wliich must be almost wholly Icamt.

SPECIATION, ECOLOGICAl AND GENETIC 307

It would be expected that simple songs would be innate,
elaborate songs learnt. But while this is true for simple songs like
the chiffehafF’s and for more elaborate songs like the white-
throat’s, the elaborate song of the blackbird is innate, and the
relatively simple song of the chaffinch has less innate basis than
the blackcap’s very elaborate song.

Members of the same group may differ radically; thus die
watrblers (Sylviinae) include all diree types, e.g. chiffehafF (innate),
blackcap (partly innate) and whitethroat (wholly learnt).

In most Oscincs the call-notes are genetically determined, but
in die whinchat [Saxicola rubetra) and several finches some or ail
must be learnt. A goldfinch kept with a budgerigar developed
call-notes entirely of budgerigar type. In all other groups of
birds, the call-notes are wholly innate.

One might further expect that learnt songs would be more
variable in nature dian innate ones; but this docs not seem to
be the case (except possibly for the chaffinch).

The need for distinctiveness gives a possible clue to the origin
of this extraordinary phenomenon. Granted die widespread
capacity to imitate the notes of other species, which appears to
be widespread among Oscines (diough to a very varying degree),
the character of a song could be much more rapidly altered
modificationally, by learning from exceptional performers, dian
genetically; and this would be advantageous with two related
species, originally with very similar songs, inliabiting the same
area (cf. p. 289). The new learnt type of song might later be
rendered partly or wholly innate by mutation (organic selection;
pp. 304, 523).

Barking in dogs and its absence in wolves arc both non-gcnctic
(Iljin, 1941) ; in certain conditions dogs cease barking, in others
captive wolves begin barking like dogs.

In fungi, no conditioning mechanism (p. 303) can operate, as it
obviously depends on a liigh degree of nervous specialization.
Recent research (T. Johnson and Newton, 1938) on the wheat
form oiPuccinia graminis (sec p. 301) show that the inbreeding (by
selfing) of biological races brings about the appearance of many
new types, apparently by the bringing to light of mcndelian

3o8 evolution: the modern synthesis

recessives. Some of the new characters concern colour, others
are semi-pathological, while still others cause a change in the
life-cycle or an alteration in virulence. The authors point out
that homozygosity in rusts like Puainia must be rare. Thus
the variety of biological races would here appear to be main-
tained throi^h a wide range of variability for those properties
concerned with adaptation to various hosts, coupled presumably
widi widespread mortality of the non-adaptive combinations.
If so, then we have a close parallel to the method by which
ecodines are established in higher plants (p. 275), and another
case of marked divergence between plants and animals as to the
mechanisms underlying adaptive difierentiation.

True physiological races do occur in parasitic higher plants,
e.g. the common mistletoe Viscum album (see A. W. Hill, 1930).
Here, however, the mechanism of differentiation seems to be
similar to that in insects. This species comprises three main races
or groups of races, one parasitizing deciduous trees, one firs
(Abies) and one pines (Pittus). These are so strongly differentiated
that the seeds of a fir misdetoe, for instance, will not grow on a
pine or vice versa: Adsible diferences between the races, however,
arc neg%ible.

5. PHYSIOLOGICAL AND REPRODUCTIVE DIFFERENTIATION

Biological races provide the best-analysed cases of evolutionary
divergence which is whoUy or primarily concentrated on physio-
logical as opposed to morphological characters. However, there
are numerous other examples. E.g. subspecies of ratdesnakes may
show marked differences in toxicity of venom (Baily, 1941) :
see also p. 273. We have also referred to the preponderance of
vocal divergence in ecologically differentiated species-pairs of
birds with inconspicuous habits (p. 289). Geographical differen-
tiation in song is, however, quite a general phenomenon in
birds. For instance, Promptoff (1930) and Howard (1900, 1902)
have studied the geographical variation in the song of the cliaffinch
(Fringilla coelehs), and find it quite marked. An interesting point
stressed by Promptoff is that the characteristic differences in the

SPECIATION, ECOLOGlCAt AND GENETIC 309

song of chaffinches arc in part learnt by the young birds (Heinroth
considers them entirely learnt; p, 306). Thus the different
geographical groups will tend to maintain their song-difFcrenccs
in spite of a considerable amount of exchange of populations
through the wanderings of young birds — a rather special example
of the principle of organic selection. Howard also noted geo-
graphical variation in the song of several other species. In
general, he concludes that a more humid environment is corre-
lated with a lower pitch. In some species, e.g. blackbird {Turdtts
memla), cuckoo {Cumlus canorus), great tit (Parus major) and
sedge-warbler {Acrocephalus schoenobaenus), he found marked
geographical variation, while in others, such as yellowhammer
[Emberiza citrinella) and cole tit {Parus ater) it was slight, and in
still others, e.g. willow warbler {Phylloscopus trochilus), he could
detect no differences.

The Shetland subspecies of wren {Troglodytes t. zetlandicus)
differs more obviously in song than in size or colour from the
type subspecies, while the reverse is true for the St. Kilda form
(T. t. hirtensis). We have mentioned the vocal divergence of
crickets (p. 298). Without doubt similar phenomena await dis-
covery in all groups in which sound is concerned with sexual
recognition or stimulation.

The recent intensive field study of birds has also brought to
light many interesting examples of biological differentiation in
habits. Thus the common robin {Eritkacus rubecula), which is
proverbially tame and an associate of man in Britain, elsewhere
in its range frequents quite other habitats and may exhibit a very
different temperament. For instance, in many parts of central
Europe it frequents pine woods, and is not specially tame. In
fact Heinroth and Heinroth (1924-1926; vol. I, p. 10) arc sur-
prised at what they regard as the legend of its tameness, and say
that robins in nature arc almost invariably shy and suspicious.
Mr. H. F. Witherby informs me that both in .Spain and in
Corsica it prefers woods remote from human habitation, but
whereas even in these situations it is tame in Corsica, in Spain
it is very shy.

Tameness may be genetically fixed in regions where normal

310 evolution: the modern synthesis

predators are absent- Thus in the Galapagos islands Mr. D. Lack
tells me that a tyrannid flycatcher, Myiarchus, hopped all over
him, endeavouring to remove hair from his head, beard, and
armpits as nest-material; and Beebe (1924, p. 285) records that
the local buzzard (Bt/feo galapagensis) can be approached to
within two feet, and specimens have been captured in a butterfly-
net!

Individual and local pecuharities in such habits as nest-building
have been noted in many species of birds. As an example we
may cite Herrick (1939) on the American robin (Turdus migra-
torius). Thus in New Hampshire and Ohio the species never uses
leaves in the construction of its nest, in spite of their abimdant
availability. In New England, where leaves are employed, a
particular individual showed a marked preference for those of
the silver maple. While these differences appear to be- genetically
determined, others depend on the availability of particular
materials. Thus in the northern part of its range, where the
birds are confined to stunted spruce woods, they construct a
dense large frame of spracc-twigs, moss, and lichens, and are
driven to use grass-blades or moss as lining in place of the cus-
tomary mud or clay. In northern Maine, twigs are employed for
the frame in place of the customary grasses and weeds, and leaf-
mould for the hning. Such difterences in nest-construction,
dependent on availability of material, provide yet another
example of organic selection. Genetically determined preferences
are hkely to be selected for later, to accentuate and fix the differ-
ences imposed by the environment.

The choice of nest-site itself may be changed by the environ-
ment. Thus to take only a few from the wealth of possible
examples, on the treeless island of Texel ofiT the Putch coast,
kestrels {Falco timmculus) breed on the ground instead of in
branches (Van Oordt, 1926), and stockdoves {Coltmha oenas)
in holes in the ground instead of holes in trees. Cormorants
normally breed on rocky ledges; but in various places they have
taken to nesting in trees. All such differences in habit, while
originally mere modifications, afford a basis for further genetically-
determined divergence of an ecological type. J. Fisher (1939(1,

SPECIATION, ECOLOGICAI. AND GENETIC 31I

CL ii) gives various examples of this plasticity both in regard
to nest-site and nest-material. Instinctive habitat-selection tends
to isolate bird species ecologically, but Lack (19406) considers
that it plays little part in primary spcciation, though it may
help to keep differentiated forms from meeting and inter-
crossing (see p. 254).

Many birds occasionally lay eggs in the nests of other species.
This aberration of reproductive instinct has without question
formed the basis for the evolution of the various cases of repro-
ductive parasitism seen in cowbirds, cuckoos, etc. Cuckoos may
show further differentiation into strains adapted to different
fosterer species (Jourdaiii, 1925). Among insects, the slave-making
ants provide a parallel example of reproductive specialization.

In migratory species of bkds, differentiation may be promoted
by individuals remaining in their winter quarters to breed, and
eventually establishing isolated breeding-groups. Among examples
cited by Meinertzhagen (1919) are the breeding colonies of the
bee-eater Merops apiaster in S. Africa, and of the sandpiper
Tetanus hypoleucus in E. Africa. The common European swallow,
Hirundo r. rustica, is suspected of breeding in Uganda. However
established in the first instance, such groups would be repro-
ductivcly isolated and might readffy come to show visible
diferentiation.

A remarkable combination of biological with geographical
divergence is seen in Trichogramma (summarized in Thorpe, 1940).
The American forms of this hymenopteran egg-parasite have
been carefully studied, and prove to be characterized by bio-
logically important differences in length of life-cycle (due to
differences in temperature-optimum), accompanied by slight
colour-differences. Though the various forms are primarily
geographical, there is considerable overlap of distribution, proving
the existence of some physiological or reproductive barrier to
intercrossing.

The biological differences are highly modifiable by environ-
ment,; so that rearing under standard conditions is needed to
demonstrate them. The differential diagnosis of natural forms is
consequently a matter of extreme difficulty. One authority on

312 evolution: the modern synthesis

the genus is reduced to describing a certain form as that “which
has distinrtly lemon-yellow females during the warm part of
the active season” !

This, like most cases of true biological races, has not been
brought to light by taxonomists, but by workers in applied
biology. It is die economic importance of divergence in physio-
logical characters in pests and counter-pests which has led to the
discovery of this new type of taxonomic diversification.

An interesting physiological divergence occurs in the termite
Formenkreis Nasutitemes gmyanae (Emerson, 1935 )- This can be
divided into two distinct groups (considered by Emerson as good
species) according to whether the nests contain one or anodier
set of staphylinid beetle species as nest-parasites. The distinction
is absolute, and is correlated with sHght differences in die soldiers’
head-size, though the sexual forms are indistinguishable. Emerson
(1934) also reviews cases where termite speciation is accompanied
fiy speciation of the contained protozoan symbiotes.

An even more curious case is that of die leaf-hopper Cicadulina
mbile, which is divisible into two races solely on ability or inability
to transmit the virus of “streak disc^e” in maize (Storey, 1932).
The difference in this case depends on a single gene (a scx-lhiked
dominant), and is concerned with the penetrability of die gut-
wall by the virus. Here we would seem to have an “accidental”
character present for unknown reasons in dimorphic balance
with another. It is easy to see how, through the effect of the
virus on the food-plant, it might become the basis for adaptive
biological differentiation. But the two forms cannot yet be
regarded as tirue biological subspecies.

Numerous infra-specific groups differiig in life-cycle and
reproductive mechanism also exist. Thorpe (1940) gives examples
of these. Thus the spurge hawk-moth comprises some iidividuals
which are subject to an obligatory diapause in development,
while others do not; some authorities maintaui that the cock-
chafer is divisible into groups characterized by tlirce-year and
four-year life-cycles; and so on.

A peculiar type of differentiation has been analysed by dc
Larambergue (1939, 1941) in die pulmonate mollusc Bulims con-

SPECIATION, ECOLOGICAL AND GENETIC 313

tortus. Here some specimens lack a penis, and are therefore
obligatorily self-fertilizing. In certain localities such aphallic
individuals constitute the vast myority of the population, in
others they are almost absent. Aphallism has a genetic basis, but
artificial selection in the laboratory has as yet been uijable to
produce stocks in wliich all individuals either lack or possess a
penis. The condition may possibly be one of balanced selective
advantage, comparable with that of gynodioecism in plants
(p. 140; and see Mather, 1940).

Reproductive divergence as a cause of speciation is discussed
by Hogben (1940). It may characterize related species. Thus
in five species of the sea-anemone genus Sagartia five distinct
methods of reproduction exist (Stephenson, 1929). It may
also be of preadaptive advantage. Then we have the recent
very rapid extension of range in the gastropod Paludestrina
(Potamopyrgus) jenhnsi in the fresh waters of this country, while
elsewhere it seems to be restricted to brackish waters (Robson,
1923). Later work (Sanderson, 1940) shows' that both British
and continental types are parthenogenetic. The British form,
however, appears to be tetraploid, and this may be the cause of
its greater ecological tolerance.

A somewhat similar case, but one in which the reproductive
advant^e seems to be causing the replacement of one type by
another, not the extension of range of the species as a whole, is
described by Crosby (1940) in the primrose. Primula vulgaris.
In this normally heterostyled species, long homostyle plants
(with pm style and thrum anthers) have been foimd in abun-
dance in an area in Somerset, the abundmee decreasing round a
centre. If, as seems to be the case, these homostyle plants are
normally self-fertUized, it can be calculated that, owing to the
peculiarities of reproduction in heterostyle forms, the homostyles
will increase at the expense of the two normal heterostyle
types (see p. 222).*

* The problem remains as to why the homostyle condition has not everywhere
become normal, since occasional homostyles are fbimd, presumably as mutants,
in numerous natural populations. Possibly the homostyle type which has become
abundant is exception^y fertile. In any case hcterostyly has a long-term
advantage in promoting out-crossing (see p. 107).

314 EVOLUTION: THE MODEMN SYNTHESIS

Various cases in insects are known in which geographical
races differ in reproductive methods. Thus in Diprion polytomum
S. G. Smith (1940, 1941) finds that, though all races are capable
9f parthenogenesis, in some the. unfertilized eggs produce
only males, in others females plus a few functionless males. In
addition, the types differ in chromosome-number, so that the
divergence is also a genetic one. There are some other physio-
logical differences, but no morphological distinctions. Vandel
(1939) finds that in a woodlouse of the genus Trichoniscus, the
rriploid parthcnogcnetic form is much more resistant to low
temperature and aridity than the diploid sexual form, and has a
correspondingly wider dktribution.

The triploid and autotetraploid varieties of numerous plants
and some animals also fall into this category. These often differ
in consequential characters affecting size, vigour, temperature-
resistance, etc., and often in the prev^ence (obligator)' in triploids)
of non-sexual methods, of reproduction (seep. 335)-

We have already mentioned the primarily physiological differ-
entiation of the forms of Carabus nemoralis (Krumbiegel, 1932),
the physiological chararters of the geographical races of Lyman-
tria {p. 216), and the geograpliical differences of temperature-
resistance in Drosophila ftmebris (p. 191). Similarly, on the species
level, the North American grape Vitis labrusca is much more
cold-resistant than the European grape, and crosses with it can
be used to confer cold-resistance on wine-grapes to be grown
in climates with low winter temperature (Wellington, 1932).
Eloff (1936) has shown that local genetic ^fferences occur in
Drosophila melanogaster as regards the tropisms of pupating larvae,
those from a certain area in S. Africa pupating on or in the wet
culture medium instead of creeping up to a dry situation.

The differentiation of the genus Gammarus seems in many
cases to have been primarily physiological, in relation to saUnity.
A salient example is G. tigrinus, recently described by Sexton
(i939)- hs morphological differentiae are quite slight, but it is
characterized by an exceptionally high range of tolerance for
salinity and dissolved substances in general, which results in its
being restricted in nature to inland waters of peculiar composition.

SPECIATION, ECOLOGICAL AND GENETIC 315

The case of G. zaddachi (Spooner, 1941, and sec J. Mar. Biol.
Ass., 24 : 444) is even more interesting. This is essentially a
brackish-water species, which in short estuaries in the west of
England exists only in a low-sahnity form. In long estuaries,
however, a high-salinity, form also exists, nearer the sea, and
exhibits visible differences in a few minor characters. Though
the two types are so similar, and though their zones somewhat
overlap, there is no intergradation in nature, and in captivity,
though they will mate aiid occasionally produce eggs, they are
intersterile. These are clearly incipient physiological species, but
the origin of the genetic barrier between them is as yet obscure.
It is possible that in Germany the ecological relation of the
species, and its differentiation, may be somewhat different.

The non-migratory (land-locked) and migratory forms of
salmon, as well as the non-migratory brook and lake forms and
the migratory form of the trout (Tchemavin, 1939) provide us
with another type of physiological differentiation. The distinc-
tion between the non-migratory and migratory forms seems in
some cases to have been compulsorily imposed by geographical
changes resulting in some types becoming land-locked, and
further differentiation, some of it adaptive, to have occurred
subsequently; in others, however, no such isolation can have
taken place, and the behavioural divergence must be primary.
As we have already seen (p. 282), a somewhat similar divergence
has occurred in lampreys.

“Preadaptations” which might give rise to physiological
differentiation arc probably not uncommon. Thus Cause and
Smaragdova (1939) find that the sinistral form of the snail
Fruticicola lantzi loses weight more rapidly than dextrals when
starved. A species of American salamander contains two types
differing in the size of their red blooti-corpuscles (Finn J. B., 1937 !
J. Hercd., 28: 373). The frizzled fowl (p. 1 18) provides an excellent
example occurring under domestication where the preadaptive
mutation has actually been utilized. See Chap. 8, $5 5, for further
examples.

"We may conclude with a very extraordinary example of
reproductive divergence, described in detail by Meyer (1938) after

3i 6 EVOtUTION: the modern synthesis

discovery by Hubbs and Hubbs (1932). They found a cyprinodont
fish {Mollknisia formosa) which was characterized first by being
always associated with one or other of two closely-related species,
M. sphenops and M. ktipinna, and secondly by consisting solely
of females. Investigation revealed that the eggs of this species
were activated by the sperm of the males of the other species!
The one species is thus a reproductive parasite on the others,
and mating occurs normally between it and them, though there
is no resultant true hybri^zation. M. formosa itself, however,
appears to be itself a natural hybrid, formed where the other two
species meet, and maintaining itself in this peculiar fashion.

These examples will suffice to show how widespread are
various forms of essentially physiological (non-morphological)
evolutionary divergence. Thorpe (1940) concludes that certainly
in most phyla, and probably in all, “there exist . . . groups
of individuals which are undoubtedly distinct species in every
sense except the accepted morphological one”. We have given
numerous instances showing the phenomenon in its incipient
stages. And a survey of any group will reveal many cases in
which physiological and ecological divergence must have been
primary, morphological distinctions having been added in the
course of later evolution.

6. SPECIAL CASES

In this section we shall refer to certain peculiar types of taxo-
nomic groups which do not seem to fit into any of the normal
categories of evolutionary diferentiation. The most interesting is
that of certain mosquitoes and gnats. The existence of these
groups, like that of biological races, was first detected owing to
their practical importance — ^in this case, for human health.

The intensive study of malaria had led to two apparently
opposed views as to the methods to be used in eradicating the
disease. The one, basing itself on the indubitable fact that malaria
is transmitted by mosquitoes, urged tlut the insect vector must
be eliminated; the other, adducing the equally indubitable fact

SPECIATION, ECOLOGICAL AND GENETIC 317

that improvements in housing, notably in separating stables from
human dwelling-places, often resulted in a marked drop in
malaria incidence, was all for concentrating on “bonificafon”
and the general raising of standards of living. Both have no '
been proved right— and both wrong: each method applies ont\
to certain forms of malarial mosquito.

The insect vector of human malaria in Europe is generally
stated to be a single species of mosquito. Anopheles maculipainis.
Recent work (see Hackett and Missiroli, 1935; Hackett, 1937;
Swellengrebcl and de Buck, 1938) has shown that this “species” in
reality consists of at least eight distinct groups, each with dieir
own characteristics. No structural or colour differences between
the adults of the various forms have yet been detected, or in the
pupae; the larvae show slight structural differences, which in
any case are valid only when tested statistically. But each form
can be immediately diagnosed by egg-characters, both its colour
and pattern, and the size and structure of the egg-float, and
these are completely correlated with striking differences in habits
and ecological preferences. In addition, each form has its own
characteristic distribution, though there may be considerable
overlap. Thus, to take but four of the forms, race typicus, with
light grey eggs barred with black and large rough floats, breeds
in fresh, pure, and usually running water, shows complete
hibernation, refuses to breed in captivity, and neglects man
entirely if other sources of blood (e.g. cattle) are available. It
is mainly an inhabitant of mountain ranges. Race elutus, on the
other hand, has an unpatterned egg, without floats; it breeds
in shallow stagnant waters, often brackish or quite salty, as it is
the most tolerant to salt of all the races.* In its feeding habits
it is the most strictly adapted to man, and prefers human blood
even when animals arc also present. Geographically it is a southern
form, confined to the Mcitcrrancan region. Thirdly, we have
atroparvus, a northern form with dappled eggs, and small smooth
floats. It breeds by preference in cool and slightly brackish waters,
and mates readily in small cages; it is unique in that the males

^ This race shows a further physiological subdivision, since in Palestine if
appears to lack this high tolerance, and is there coufmed mainly to fresh water

3i8 evolution: the modern synthesis

do not assemble in swarms. It winters in rather warm places,
showing only partial hibernation, and feeds at irregular intervals
during this period. It tends to have rather short wings (averaging
about 10 per cent less than in race messeae). It will bite both
man and animals, and a considerable proportion may be attracted
away from man to animals, notably pigs and horses. In HoUand,
which has been carcfuUy investigated, it and messeae are the
two races chiefly present (with typicus occasionally found in the
east). Their distribution overlaps considerably, but in regions
of more brackish water messeae is rare or absent, while in fresh
water it is in the majority, though not preponderantly so.

Finally, race labranchiae, with pale, broad eggs and very small
but rough floats, is an inhabitant of brackish and salt marshes
in warm regions. Its hibernation is both short and very imper-
fect, and it will bite man as well as animals, -with rather more
preference for man than atroparms.

As a result of these peculiarities, elutus is always associated
with intense malaria, which can only be eradicated by destroying
the mosquito or its breeding places, or preventing the insect’s
access to man. Typicus, on the other hand, is of very little impor-
tance as a malaria- vector, and raising the standard of life, by
increasing the number of domestic animals and providing separate
accommodation for them, will deviate it almost entirely away
from man. Labranchiae is a serious malaria vector, which can
only be partially deviated to animals; while atroparvus is a source
of mild endemic malaria, and can be to a considerable extent
deviated away from man by improving conditions.

The different races arc also separated by sterility barriers.
These arc in some cases complete, but the stage at which they
operate varies. Thus in some cases no eggs arc obtained, or the
larvae all die soon after hatching; in the atroparvus-ehitus and
atroparvus-messeae crosses the larvae die, but at a later stage,
while the typicus-atroparuus cross gives healthy but sterile adults.
In other cases, the barrier is only partial; sometimes all males
and some females arc sterile, in others all females and some
males arc fertile. Thus biologically these forms arc full species.

Swcliengrcbel and de Buck (1938, p. 90) have shown further

SPECIATION, ECOtOGICAL AND GENETIC 319

that even within a single race (atfoparvus) considerable diversi-
fication may exist, different strains showing dififerent ecological
preferences and different resistance to salinity, and broods occa-
sionally turning up with unusual characters of eggs or larval
hairs. They consider that many other races will show si milar
intra-group variation.

The practical needs of human health having brought these facts
to light, a similar differentiation into ecological races has been
looked for and discovered in other forms, both of Anopheles
and of the common gnat Culex pipiens (see summary in Thorpe,
1940).

Finally, it is of some interest to note that the malaria parasites,
as well as their vectors, are differentiated into physiological races.
Thm Plasmodium vivax, the tertian parasite, exists in at least two
forms (see Swellengrebel and de Buck, 1938, pp. 227 seq.) differ-
ing in number of merozoites, incubation period, type and gravity
of symptoms produced, latency, susceptibihty to temperature
and anti-malarial drugs, and in showing an incomplete reciprocal
immunity. Different strains may in some cases be capable of
hybridization within the insect vector (Manwell, 1936).

Tliis case has been dealt with at some length because of its
numerous points of interest. From the evolutionary standpoint,
the type of diferentiation is unique in that the races, while
well-defined ecologically and physiologically, and to a consider-
able extent geographically, yet show much overlapping, and are
only kept distinct % genetic (reproductive) barriers. It is for the
present extremely difficult to understand what has been the actual
cause and mechanism of their evolutionary differentiation.

Equally puzzling, though in quite a different way, is the case
of the common limpets (Patella) of Europe and North Africa,
investigated by Fischer-Piette (1935). Here, again, two apparently
contradictory opinions were prevalent, one that they constituted
but a single species, the other that they should be divided into
at least three species. Again, both views were partially right.
Fischer-Piette, on the basis of extensive collections over a large
area, has been able to show that in certain regions the assemblage
of limpets falls into three discontinuous groups, characterized

320 evolution: the modern synthesis

both structuraUy (differences in radula-tceth, etc.) and ccolo^-
cally, each having a preferred zone of the intertidjd area In
othir regions, however, no such separauon “
assemblage of limpets forms a continuous whole, diffcr<mt
types mtererading completely with each other. .

Sc has however, infornica me verbally that the a,str,b>rt,m,
Xr^y turn oat to be trimoaal, tte tee modes ot rhe
cotociLg with the modes of the three separate type, of
Xr Xs. These general eoneteion, have been confirmed
by Eshek (1940) at Port St. Maty (Isle of Man), where, how-
ever, only two types can be distinguished.

Here we would seem to have an ecological divergence, which
in some regions has led to complete speaation, ui others on y
to a partial separation of adaptive types. But whether the cm-
dition of a single continuous trimodal group is primary and
constitutes a step towards complete separation, or whether it is
secondary, resulting from hybridization of tlirec P^™^
differentLd types, it is at present impossible to
mterpretations present obvious difficulncs. So far, ^ p
menul work has been undertaken on the very interesting

’'^E^pl^mewhat recalling the state of affairs in mwquitoes
arc provided by various insects, notably Hymcnoptcra. Here we
may find “races” differing slightly in visible characters, m just
the same way as do typical geographical subspecies, and showuig
no intergradation or other signsof interbreeding, yet over apping
geographically to a greater or less extent In the oveijap area
they may be found quite close to each other, so that their dis-
tinctness cannot be brought about by spatial separation as occurs

in some cases of ecotopic divergence. _

Thus Bequaert (1918) describes “races” of die wasp Eumenei
maxillosus, characterized solely by coloiur-characters, whic , w n c
possessing characteristic geographical ranges, arc none o t cm
mutually exclusive (see map in Robson and Richar s, i 93 .
p. 68 ). A few appear to have an ecological basis, c.g. one is
coiifmcd to deserts and siibdcscrts, another to tropica rain orcst
and its neighbourhood, still auodier to typical savannahs. One

SPECIATION, ECOXOGICAL AND GENETIC 321

might be regarded as a geographical race in that it is confined
to Madagascar; but this region is shared by another form which
extends there from the African continent. Intermediates are
sporadic, and not confined to the zones where two ranges meet.
An interesting feature is the recurrence of a number of the
colour-patter»is (some of them very striking) in related species.
There is a possibiHty that this may be due to synaposematism
(Mullerian mimicry); on the other hand, it also recalls the
“homologous scries” of parallel colour-variations found in various
grasshoppers (p. 516), where genetic analysis has been possible
and has revealed the existence of a selective balance as primary
cause of the polymorphism (p. 99). Interestingly, in another
Afiricat^ wasp, Synagris cornuta, the equally striking colour varia-
tions are connected by more frequent intermediates, and several
may occur in a single colony.

The careful studies of Richards (1934) on another genus of
wasp (Trypoxylon) have shed new Ught on the subject. To take
but one set of three “species”, T. salti, T. spinosum, and T.
armatum: the first two arc extremely similar, and intergrading
forms occur. Their ranges overlap in Central America, though
T. salti extends much further south. The third form, armatum,
is more readily distinguishable, and less closely resembles salti,
with wliich it overlaps, than spinosum, with which it docs not.
As Richards says (p. 243), “the present resources of entomological
nomenclature are insufficient to deal with a group of forms such
as the^”. Salti and spinosum intergrade, so cannot be regarded
as full species; their ranges overlap and intermediates occur over
a considerable part of the joint range, so they arc not geographical
subspecies; nor are they mere varieties (aberrations) or examples
of simple ffimorphism, since “in a large part of their range each
form seems to maintain a homogeneous population”.

Richards reaches the interesting conclusion that “geographical
segregation in insects is often of a different nature to the more
famihar process observable in birds and mammals . He suggests
that non-geograpljical — ^i.e. ecological or physiological forms
of isolation can be much more effective in insects — ^a conclusion
borne out by the abundance of “biological races in the group, as

322 evolution: THE MODEEN SYNTHESIS

contrasted with their total absence, in any strict sense, in higher
vertebrates. Experimental analysis of such cases is urgently needed.

If this is correct, a special type of ecobiotic divergence, com-
bined with considerable geographical differentiation, is frequent
in insects. Richards cites other examples, e.g. in hornets, which
seem to fit in with this idea, and the case of Eumenes just cited may
depend in part on this. He informs me that the same phenomenon,
of “non-geographical speciation”, also occurs in various beetles.

These general conclusions seem to be borne out in other
insects, e.g. in ants. The numerous forms (subspecies or species
according to taste) of Myrmica rubra seem to conform quite
closely to Richards’ views, since they are very similar morpho-
logically, have distinct ecological preferences, and do not normally
cross in spite of extensive spatial overlap.

Particularly interesting is Diver’s summary (1940, p. 317) of
our knowledge concerning two forms of Lasius niger, L. n. niger
and L. «. alienus (considered by a minority of authors as good
species). Morphological distinctions between the two are very
shght, being confined to the presence or absence of a few small
hair s on antennae and tibiae. Differences in behaviour, however,
are more definite, and usually permit identification in the field.
The geographical range of alienus appears to be wholly confined
within that of niger. Ecologically, the two forms show distinctive
preferences. Niger has much the greater range of tolerance, from
sand-dunes to wet sphagnum bog, from grass to dead trees,
while alienus is (in the Dorset area investigated by Diver) almost
confined to dry heath, with some overflow onto moist heath
and turf. Even within the single type of habitat represented by
dry heath there are differences, alienus preferring blown sand.
Sometimes the two species nest only a few feet apart. In regard
to swarming dates there is an extensive overlap, providing
opportunities for crossing, though niger tends to be slightly later.
Very occasionally forms are found which are intermediate
between the two types, not only morphologically but in behaviour
and ecological preferences. These, in the absence of evidence to
the contrary, must be assumed to be produced by intercrossing,
which must accordingly be rare.

SPECIATION, ECOtOGICAI. AND GENETIC 323

We seem here to be dealing -with fine ecotopic divergence
which has just reached the stage of speciation; but it is extremely
difficult to envisage by what means the distinctness of the two
types was first brought about.

Diver gives other puzzling examples — e.g. three apparently
distinct species of hoverfly (Syrphus) with extremely small
morphological differences, with the geographical distributions
of one species including that of the other two, and of the second
including that of the third, with general similarity in ecological
preferences, and extensive overlap in the periods in which adults
are on the wing. Diver suggests that the two species with more
restricted distributions have arisen by the segregation of itmall
discontinuous groups, followed by accidental divergence and
later expansion of range, but this is quite speculative.

We must also remember the remarkable case of the two
‘races” of Drosophila pseudoohsatra (p. 369). These differ in
various physiological characters such as temperature-resistance,
are intersterile, and are further characterized by sectional chromo-
some-rearrangements which could only have originated in
isolation. They have different geographical distribution, but
overlap considerably. It seems clear that the divergence leading
to intersterihty first occurred in an isolated local group, which
later was able to inva,de the other’s range. Within each race,
“strong” and “weak” forms are found which differ in their
sex-determining mechanism like those of Lymantria, and also
resemble those o£ Lymantria in showing some gradation in their
distribution (see p. 359, and Dobzhansky, 1937, p. 284).

In any case, the taxonomic differentiation of invertebrates
clearly provides a vast and almost virgin field for experimental
analysis. A number of general principles have emerged as a
result of intensive work on various organisms, plant and animal,
vertebrate and invertebrate; but their relative share in causing
differentiation may difier markedly from group to group.

7. DIVERGENCE WITH LOW COMPEOTION; OCEANIC FAXJNAS
Decreased selection-pressure permits increased variation. This is
true not only for species or subspecies but for entire groups. In

324 EVOtUTION: THE MODERN SYNTHESIS

the former case the result is higher variability, in the latter more
extensive evolutionary divergence and radiation. An excellent
example comes from the Cichlid fish fauna of the African lakes
{Worthington, 1937, 1940; summary in Huxley 1941a). In some
of the lakes, their chief predators (the large fish Lates and Ilydro-
cyon) arc wholly absent. Where tins is so, the Cichlid radiation,
as measured by the number of endemic species, and as sliown by
the greater variety of ecological niches occupied, is far greater.
Thus of the lakes isolated during the second pluvial or later,
Victoria-Kioga and Edward-George are without large predators,
and contain 58 and 18 endemic species respectively, while Albert
and Rudolf, where the predators are present, contain but 4 and 3 j
the larger number of endemics in Victoria-Kioga is due to greater
environmental diversity. Nyasa and Tanganyika were isolated
during tlic early Cenozoic j die former lacks predators and con-
tains 171 endemic species, while the latter, where predators are
present, contains only about 90 endemics in spite of its greater
environmental diversity.

The principle can be generalized in relation to competitor-
pressure as well as to predator-pressure. This is well shown by
the Australian marsupials. These also illustrate the fact that the
total radiation of the fauna is normally not increased: among
them some of the chief placental types are missing, and various
adaptive characters, notably intelligence, arc below placental
standard. However, the best examples are found on oceanic
jclanrlc Here, the number of types which have established them-
selves is much restricted, and under these conditions of biological
low pressure, the few favoured groups may differentiate into a
surprising variety of forms.

Perhaps die most remarkable example of oceanic radiation
is afforded by the sicklebills {Drepanididae) ofthe Hawaii archi-
pelago. General accounts are given by Giffick (1932) and Mord-
vilko (1937). The Drepanididae are passerine birds, according to
Gulick derived from a tropical Amcricain honey-creeper, accord-
ing to Mordvilko from a finch related to the goldfinches (Gflr-
duelis). In any case, they arc now restricted to Hawaii and to
Laysan Island, 800 miles further west, and have produced a

SPECIATION, ECOrOGICAI, AND GENETIC 325

quite astonishing variety of types, meriting division into no
less than i8 genera. There are small insect-eaters and finchlike
seed-eaters, some with small and some with heavy bills; a very
large-billed nut-eater; a peculiar woodpecker type {Hetero-
rhynchus) with long upper mandible for prying away bark, and
short lower mandible for probing out wood-boring grubs;
nectar-suckers (with special tongues); forms wliich combine
nectar-eating with searching for insects in flowers; and others.
No other bird family shows such adaptive diversification; at
first sight one would say that half a dozen distinct families were
represented.

A characteristic of many genera is the curvature of the beak,
from which the family name of sicklebill is taken. In coimec-
tion with this, the bird-poUinated plants of Hawaii have
curved corolla tubes, while those of their mainland relatives are
straight.

Mordvilko stresses the analogy of the evolution of a group
like the sicklebiUs with the excessive radiation of domesticated
animals. Gulick points out that the bill and habits of a form
like Heterorhynchus are true evolutionary novelties, which have
not been evolved elsewhere. A similar example is afforded
by the freshwater gobies of the same area, which have evolved
unique sucker-hke fins for clinging to the rocks in rushing
torrents.

The ancestral sicklebill must have been the first bird immigrant
to the archipelago. The remaining passerine fauna belongs to
four families only — crows, thrushes, flycatchers, and honeyeaters.
Differentiation here is not nearly so marked, though endeinic
genera have in some cases been evolved; presumably these were
all later arrivals.

A very similar case is that of the groundfinches (Geospizidae)
of the Galapagos. Here, again, we have a family confined to an
oceanic arcliipelago and an outlying island (one monotypic
genus, on Cocos Island). This family is highly differentiated
(five well-marked genera on the G^apagos); the remaining
passerine birds are much less distinctive, though endemic genera
have been produced, and must have arrived later than tlie ancestral

326 / tee mobein: synthesis ,

.gsospizid, wkn there was Bot OEly less time available, biit-many
niches had been filled. In most archipelagoes, however, mmy
endemic forms are differentiated, they do not mually
geographically. However, in the Geospizidae (Eke the sicklebills}
several distinct species (up to loin the groundfinches) may coexist
on one island.

The Geospizidae have been subjected to an exhaustive taxo-
nomic analysis by Swarth (i 934 )» while Lack (1940^) has been
able to draw important general conclusions from-liis study of
the group in the field. Most Geospiz/i species are large-biUed
and eat seeds, some having the most powerful biEs of any
passerine birds , Platyspiza is mainly a leaf-eater, Camarhynchus
.mainly insectivorous, while Cactospiza has evolved from Cama-
fhynchus in the direction of a woodpecker: it also has the unique
habit of using a twig as a tool to pry out insects, thus making
up for the incomplete specialization of its beak. Finally Certhidea
resembles a warbler both in beak and habits.

In addition to this ‘^minor adaptive radiation , as Lack calls
it, numerous non-adaptive specific differences exist, presumably
due to the Sewall Wright effect (p, 58). In addition, some of
the beak differences are concerned not with adaptation to mode
of life, but with specific. recognition for mating purposes. Lack
concludes that the virtual absence both of competitors and of
predators has permitted this remarkable radiation. Elsewhere an
island lias been colonized by two closely-related species which
will not interbreed (p. 255), Apparently the rapid differentiation
of the Geospizids in peculiar conditions has permitted this
phenomenon to be intensified. Large-scale hybridization docs not
seem to have contributed (sec p. 356),

He also draws attention to the partial or total loss by many

* The interesting point may here be mentioned that insular land birds tend
to have larger bills (cither longer or more robust) than their nearest continental
relatives (Murphy, 1938), This applies to non-oceat)ic as well as to oceanic
islands. Murphy has checked this statisticaHy for North American passerine specks
and finds tiiat it holds for all insular full species, and for 78 per cent of die insular
subspecies. Chapman (1940) confirms the fact for central and South American sub-
species of the sparrow Zmotrichiacapensis, The enormous bills of some Ceospiza
species on the Galapagos may illustrate the same phenomenon. Its significance
is at present quite unknOwa,

■ SFECIATION, ECOLOGXCAX . AND GENETIC , . 327

of the species of the typical male plumage, the juvenile plumage ,
type being prolonged into the adult phase. This is a common
tendency in the land birds of oceanic islands. As another example
we may cite the hen-feathered subspecies of bullfinch {Pyrrhula
p, murina) found on the Azores (Morphy and Chapin, 1929).
This Lack -suggests to be due to the absence of related forms
with which a female might hybridize: the need for specific
distinctiveness, which is such a feature of secondary sexual char-
acters, then disappears. In the Geospizidae, as differentiation has
proceeded, distinctiveness has been reacquired in respect of the
lion-sexual beak characters. The converse of this process, as
D. Lack suggests, in an unpublished paper which he kindly
allows me to cite, may be seen in such forms as the ducks and
some pheasants, in which, owing to the looseness of the bond
between the mated pair, there is an unusual tendency to natural
hybridization. Here, the females of related species are often very
similar, demonstrating close relationship, but the males show
strikingly distinctive characters.

Other groups, too, show increased radiation on oceanic islands.
As Buxton (1935) says, ‘fone characteristic of the [insect] fauna
of such an archipelago as Hawaii is the development of complex
groups, many of them containing a very large number of
closely related species’’. Other areas with a less lengthy history,
such as Samoa, show the same phenomenon, but to a lesser

As Gulick points out, the difficulties of immigration make the
land faunas of oceanic islands ‘^disharmonic”, in the sense that
they lack the normal balanced ecological diversification of types,
being restricted to types pre-adapted to long-range dispersal
across salt water, and to a chance assortment of these. But the
longer the fauna persists, the greater will be the tendency for it
to become secondarily harmonic, through adaptive radiation of
the earlier immigrant stocks. Thus oceanic faunas represent very
peculiar special cases, but at the same time they conform tq the
general rules of evolutionary divergence.

As we should expea, precisely similar phenomena may occur
in lakes which have been long isolated. The most striking example

328 evolution: the modern synthesis

is Baikal, the zoogeography of which is discussed by Bei^ (i 93 s)-
Here the Gammarids show excessive radiation; e.g. the one
genus Echinogcmmarm is repre^ted by some 40 species, and the
three species found elsewhere are probably examples of conver-
gence and should not be placed in this genus, which then would
be endemic to the lake. In the fresh-water oligochaete genus
iMmprodilus, 12 of the 16 species are found only in Baikal.

8. genetic divergence

Next we come to a group of several methods of species-formation
which have this in common, that the primary separation of the
new type is not spatial but genetic. A iurther common feature
is that our knowledge of all of them is quite recent.

First we may take genic separation. In maize, two strains
have been found, differing only in a single gene-pair, which
will not cross. This shows that a single mutation may effect the
separation of one interfcrtile group into two. Such occui^tences
appear to be very uncommon; and for the moment the evolu-
tionary bcarmgs of this fact are not clear. We can only say that
single gene-mutations, if they affected either mating-reactions
or the delicate machinery of meiosis, might be of importance
in breaking up animal species also. Dobzhansky (193 7 . P- 263)
has a discussion of genic effects on reproduction; see also Stem
(i93<5).

Wc mention elsewhere how the randomness of mutation
will lead to interstcrility in isolated groups (pp. 1 86, 360) : but here
the genetic differentiation is secondary. A similar example is given
by Wolf (1909; and see discussion in Jennings, 1920) for Myxo-
bacteria, where non-sexual fusion of colonies from a single strain
occurs. But after prolonged culture, substrains incapable of
colonial fusion may be produced.

Most genetic separations, however, may be called chromo-
somal, as they are concerned with alterations not in genes, but
in their chromosomal vehicles (see Darlington, 1931, 1940).*

* See Dariingtoti and Upcott (i94i5) for a discussion of variation in types
of breakage and reunion of chromosomes to be found in different forms.

SPECIATION, ECOLOGICAL AND GENETIC 329

First come those cases in which the barrier to crossing, whether
more or less complete, does not produce any visible differentia-
tion, so that any taxonomic divergence follows later, as with
geographical isolation.

We may begin with segmental interchange (reciprocal trans-
location) between different chromosomes, as described on p. 90.
Here the same gene-complex is merely rearranged. The various
“prime types” thus produced can cross with each other, but
owing to peculiarities of the chromosomal mechanism tend to
maintain themselves; for the heterozygotes are less fertile than
the homozygous types, and further, crossing-over between
chromosomes which have interchanged segments is restricted so
that recombination is almost abolished. Further differentiation and
separation of the prime types into subspecies or species could
then occur by the accumulation of different mutations in different
types, and by the development of other barriers to crossing,
which would be advantageous (as preventing waste) if two or
more types occurred together.

In Datura, only the first stage has been reached; different prime
types occur in different regions, but are not visibly distinct.
This, it is probable, is due to D. stramonium having in recent
times spread rapidly as a weed of civilization, so that insufficient
time has elapsed for diferentiation.

A quite different development, however, may occur if re-
cessive lethal mutations occur in both interchange chromosome-
groups. In that case, the homozygotes will be inviable and only
the hybrid will survive. This is the condition of balanced lethals.
Since lethal mutations are common, and since the heterozygote
will enjoy increased advantage in various ways as soon as one
homozygote has become inviable, we may expect this condition
to develop out of segmental interchange at least as readdy as
that of differentiated prime types.

The classical case is that of the evening primroses (Oenothera).
Here abundant genetic and cytological evidence converge to
show that almost all the species are balanced-lethal heterozygotes,
the origmal pure types having disappeared. Elaborate subsidiary
mechanisms ensure the production of the heterozygotes with the

dons wmcii iie descriDea were hoc inutauuxw
strict sense of substantive changes in the germ-plasm, but rnerel}*
‘reconibinatioiis of a peculiar sort, to be expected only in
balanced-lethal heterozygotes, and due to occasional crossing-
over. Gene-m utations of this order of magnitude do not seem
to occur.

Blakeslee and his school have been able to produce various
interchange types artificially by X-rays, and then, in certain
cases, by means of crossing to synthesize quite new strains which
possess certain sections of the gene-complex in duplicate as
compared with the normal. These show numerous character-
differences from the type, and can be regarded as artificial
incipient species (Blakeslee, Bergner, and Avery, 1936).

Translocations, both reciprocal and non-reciprocal, also occur
quite frequently in Drosophih, and will normally produce some
reduction of fertility in the Fi hybrid (sec Stern, I 93 ^» ^

discussion of the different possible types of translocation). This
may be the first step towards speciation, though it is apparently
much less important than inversion in this respect (Dobzliansky
and Tan, 1936)*

The next method of chromosomal separation is inversion
(p. 91), Here, too, a mere rearrangement of parts of the chromo-
some outfit has occurred, but in this case by a reversal of a portion
of one chromosome, so that the order of the genes is here inverted.
Affairs are here sightly compicated by the fact that such inver-
sions often produce a visible * position-effect*’ (p. 85), This is
presumably due to genes exerting some of their effects in virtue

SPECIATION, ECOLOGIGAL AND GENETIC 33I

of a special type of interaction with their immediate neighbours:
the genes at either end of an inverted section will of course he
interacting v/ith new neighbours.

Inversions were first inferred from genetic analysis. Later they
were detected cytologkally at the prophase of meiosis in maize.
To-day, thanks to the discovery of the giant chromosomes in
the salivary glands oi Drosophila, they need not wait to be detected
by their abnormal behaviour at crossing-over and its results,
but can be directly observed; for chromosome-segments can be
seen in which the normal band and hue pattern is reversed,
and these are then found to cause abnormalities of pairing
and crossing-over at meiosis when opposite a non-inverted
segment.

Among the properties of inversions is that they interfere (of
course in the heterozygous condition only) with chromosome-
pairing and crossing-over, and it is in virtue of this fact that
they exert their effect on breaking up species. But this effect will
be quite different according to the magnitude of the inverted
segment, When it is very small, the disturbance will be small;
and crossing-over between genes one or two units apart is in any
case of very rare occurrence. Thus the main effect of very small
inversions will be vi& their visible position-effects, which. will
be similar in magnitude and nature to the effects of small true
gene-mutations. Small inversions will thus merely add to the
internal variability of a species, and will not tend to break it up
into separate groups.

Large inversions, on the other hand, will have two important
effects. They wiU reduce the fertihty of hetero2ygotes, so that
the pure types — that with two normal chromosomes, and that
with two chromosomes both with an inverted section — will be
at an advantage: and the impossibility of crossing-over between
an inverted and a non-inverted section will effectively isolate
these two regions of the gene-complex ficom each other. Recom-
bination can no longer take place between them, so that any
mutation taking place in an inverted section cannot be trans-
ferred to its non-inverted homologue, or vice versa. Darlington
(1937) was the first to grasp tiie full implications of this

332 evolution: the modern synthesis

fact, and to point out that such “chromosomal

was of equal importance with other more obvious ^mds of

isolation, such as that due to geographical separation {y. itijra;

^tinek inversions may dms cause a certain rcductimi of hybnd
fertility bctweai types. Fertility would be still further reduced
by furdicr inversions in other kinds of chromosomes, while two
or more inversions in each of two or more chromosomes would

produce very considerable sterility (Stem, i93*^)'

A large inversion may thus pave the way for*thc separation
of a species into two non-interbreeding groups. For one thing
its isolating effect may be accentuated by further inversions;
and in addition, once any degree of hybrid sterility has o^urred,
natural selection will operate to produce other stenhty barriers,
in the shape of different- mating reactions, so as to prevent the
waste caused by crossing, with its production of relatively infertile
individuals. Still further genetic sterility-barriers arc also likely
to arise by gene-mutation of various kinds. Inversions may also
lead to the production of visMe diversity by the accumulation
of different mutations in homologous inverted and non-inverted

sections. ,

If several favourable mutations ocepr in an inverted section,

and are therefore prevented from crossing-over and recombina-
tion with the homologous non-invcrtecl section, the spread of
chromosomes with the inverted section will be favoured by
selection. The two homologous sections, inverted and non-
in verted, wilTin fact each become an isolated partial genetic
system; within this there will operate the same phenomenon of
mutual genic a<^ustmcnt discussed for total genetic systems in
chap. 3 (see also Malmovsky, 1941) - These segmental harmom-
ously-stabilizcd gcnc-compleiccs will continue to evolve within
the less thoroughly stabilized total gcne-complcx. Something of
this sort has undoubtedly occurred in the divergence o( A^mu ■
saiip^ from A* fatua znd Triticutn pulgciw from T. spelta: in either
case the two members of the pair differ essentially in a group
of characters all located in a region of one chromosome, whidi
in one species has suffered inversion (Huskins, 1927, 1928).

SPECIATION, ECOLOGICAL AND GENETIC

333

In general, it appears that sectional rearrangements are rarely
if ever the sole cause of evolutionary divergence (cf. MuUer,
1940). For one thing they have a negligible prospect of becoming
established, except by chance in a small and relatively weU-
isolated group. And in the second place their presence in non-
interbreeding groups is normally accompanied by numerous
single-gene difierences, which are often responsible for much
of the group-incompatibility. They can therefore only be regarded
as secondary agents in bringing out speciation, though their role
may be quite important in species which are normally broken
up into small isolated population-units, and still more in those
(among which various species of Drosophila are to be included)
which are subject to violent fluctuations in numbers with small,
isolated groups at the low point of the cycle.

It is, for instance, probable that inversion has had a good
deal to say in the separation of Drosophila melanogaster
and D. simulans, though the single-gene e&cts must have
played the major role, since important effects on the sterility
of the hybrids are determined by them (Stem, 1936; and

p. 359).

Whenever inversion has played an important part in species-
formation, the two species may be expected to remain very
similar in appearance, since they will overlap in their ranges,
and will both possess almost the same genetic constitution, well-
adapted to a common environment. A gradual ecological diver-
gence may occur later. The same will apply to cases of divergence
initiated by translocation. Thus the two “races” (incipient species)
of Drosophila pseudoobscura can only be distinguished by statistical
analysis, which brings out, in males only, certain diflferences in
wing and leg measurements, and in the numt^rs of the teeth on
the sex-combs (Mather and Dobzhansky, 1939).

Such species wiE only be detected by refined and detailed
systematic methods, and will often not be recognized by sys-
tematists who are not ahve to the implications of genetics. It
will be of great interest to discover whether spedes-pairs of
this type occur in higher vertebrates.

A peculiar method of forming new types is that of asexual

334 evolution: the modern ■ SY,NTfiE,SIS' ,

segregation in certain parthcnogpnctic plant species of hybrid
origin. Parthenogenesis in such cases is due to a suppression o^
the reduction of the chromosomes. But even in the absence of
reduction, any corresponding chromosomes from the two
original parents which are able to pair will be subject to crossing-
over, since we now know that crossing-over takes place by ^o
stages, and not only as an accompaniment to redu^on. And
such crossing-over will produce new types, which will main^
themselves, subject to selection, save for further cross-overs (sec

also pp. 3 S 2 - 3 )- ^ .

Such a process diould lead to the formation of numerous
closely-related true-breeding types. Some of these will doiibtless
be at a disadvantage and v/ill disappear, while others will main-
tain themselves. Further divergence between the types may occur,
though slowly, by gene-mutation. It is probable that some of
the numerous species ofhawkweed {Hieracium) and blackberry
(Ruhus) owe their origin to such asexual segregation. Apparent
mutations due to this process liave been detected in both forms
(Darlington, 1937, pp- 296, 475 . for Hieracium; Crane and
Thomas, 1939. for Rubus; and p. 352 of tliis volume).

, ‘ Next wc have the various phenomena of polyploidy in which
a miiltipBcatson of whole chroiiiosoiiic-scts occurs (see P.T 43 ),

: 'As: already mentioned, polyploidy is of two fundamentaEy dis- ■
■ tinct types: which the,' chromosoiiie 'setS'^are

. ah of the same kind, derived from the same species, and initial
■ : allopolyploidy, in, whidi Aey arc of different kind, derived from
' two distinct: species*. The actual doubling is in both cases due to
the suppression of division - of a cell after^divisioii of the ehtonio-
somes 'has ' ta^ place, but whereas this is the primary e vent in
''autopolyploidy, in, allopolyploidy it is subsequent to hybridization*
As previously mentioned, polyploidy is widespread in plants,
but veryTare in animals {pp*T40 seq.). We dierc pointed out the
prieri reasop for its 'non»<xistence ,m forms. M,. J. D.

: WMte points-, out that this. might 'be expected not to apply

in liermaphrodite animal' groups.' However, his investigations o.t
chromosome-number show .that in one such group (pulmonate
molluscs) it appears not to occur at all, in another (Rhabdocoela)

SPECIATION, ECOLOGICAL AND GENETIC 335

it occurs to a moderate extent, and in two others (Hirudinea and
Oligochaeta) the meagre data suggest its possibility as a rare
phenomenon. His conclusion is that polyploidy has not occurred
in the hermaphrodite groups of animals to anything like the same
extent as in higher plants. This may be due to the rarity of self-
fertilization in hermaphrodite animals, or to some as yet unknown
cause. Polyploidy may possibly occur in Hemiptera (p. 370).

In this section, only autopolyploidy concerns us, as allopoly-
ploidy connotes convergence, not ivergence. Chromosome-
doubling wiU usually occur through failur^e of cell-division but
not of chromosome division. If the tetfaploid cell forms all or
part of a growing point, a totally or partially tctraploid shoot
will result. Such a shoot wiU not be fuEy fertile, since at meiosis
there will be four of each kind of chromosome instead of two,
so that in addition to pairs, groups of three and four chromosomes
vrill be formed. Many garhetes will, therefore, not possess two
entire genomes, but will be unbalanced, with some chromo-
somes represented in tripKcate or only in single dose; and such
gametes will often be inviable.

The continuance of the species can be ensured either by con-
emtrating on asexual reproduction, or, if fertility is not much
reduced, by means of a differentiation of the chromosomes,
presumably through mutation, so that instead of four similar
members of each kind, two sHghtly dissimilar pairs are found.
Instead of AAAA,BBBB, etc., we would have AiAijAzAz,
BiBi, B2B2, etc. So long as the dissimilarity is sufficient to
prevent pairing between members of different pairs (e.g. Ai
and A2, or Bi and B2) complete fertility will be restored. In
this case the initial autopolyploidy will have been converted
into a secondary functional allopolyploidy (p. 143; Darlington,
1937, his pp. 183, 226).

Doubling can of course be repeated, leading from tetraploid
to octoploid and higher forms. This is likely to occur especially
in types which have specialized in parthenogenetic reproduction,
leading to the establishment of series with 2n, 4n, 8n, i6n chromo-
somes.

Autotriploid (3n) forms may arise from diplpids by fertiliza-

336 evolution ; the modekn synthesis

tion between a diploid (unreduced) and a haploid (normal
reduced) gamete. Triploids are sexually sterile and can only
reproduce by non-sexual methods. Hexaploids (6n) may arise
in similar fashion from 4n plants. They may also originate by
doubling in a triploid form, and then of course reacqmre sexual
fertility. An interesting case of this sort is given by Pcrlova
(1939). The wild triploid and sterile potato species Solan^
pallis-mexici was grown at high altitudes, and there, pr^umabiy
as a result of the low temperature, produced a fertile hexaploid
form. As this species is more resistant to frost and drought and
certain diseases than other potato species, this result should be
of considerable importance.

Darlington (1937, p. 216) gives a table of autopolyploid species
and mutants. Various wild triploid species are known, all repro-
ducing vegetativcly, e.g. m tulips and narcissi. Extremely few
cases are known in animals; e.g. the land crustacean Trichoniscus:
here reproduction is parthenogenctic (p. 314; Vandel, 1937)*

Other triploid types have been experimentally produced by
crossing 2n and 4n forms. Of great interest is the fact that auto-
polyploidy may give rise to forms which are not associated with
any systematic visible differences. For instance. Anemone montana
occurs in diploid (an), tetraploid (411), and hexaploid (6n) forms,
Sikne ciliata in an. 4n, and i6ri forms, all similar.*

In other cases, the polyploid forms differ slightly in visible
charactcB, but are still classified within the limits of a single
species. There are, for instance, five such forms of Viola kitaibeliana
(some of them ancuploid) and of Prunus spinosa, and three of
Erophila {Draha) vema. Such forms wete at one time included
under tlic term ‘xlementary species’*.

The most interesting evolutionary fact concerning autopoly-
ploids, however, is that different members of a series may and
often do have different geographical distributions. In general,
tetraploid forms seem better adapted to difficult environmental
conditions. Many are more cold-resistant than their diploid

SPECIATION, ECOLOGICAL AND GENETIC 337

relatives. Accordingly we find many tetraploid forms in the far
north and in mountain regions. Almost all the grasses in Spits-
bergen are polyploids (see Haldane, 1938). The sharper dimatic
zoning produced by the glacial period. must have encouraged
the survival of tetraploids and promoted the formation of tetra-
ploid subspecies. Others axe adapted to the extreme temperatures
and great aridity of deserts — e.g. various forms of Enagrostis in
the Sahara (Hagerup, 1932). Tetraploid forms are also in many
cases more generally vigorous, a fact reflected in their distri-
bution, which is frequently wider (often considerably so) than
that of the diploid variety. Many widespread weeds of cultiva-
tion and waste land are also tetraploid forms, the diploid types
having quite restricted distribution. (For general discussion see
Miintzing, 1936, Tischler, I 94 i-)

In experimental tetraploids in tomatoes, Fabergd (1936) finds
that the tetraploid is less variable phenotypically than the diploid.
This cannot be due to ‘diminished segregation of recessives;
Fabergd suggests that it is due to the greater effectiveness, in
certain cases, of four as against two homologous genes, resulting
in greater stability ofearly developmental processes. In colchicine-
induced tetraploidy, Badenhuizen (1941) finds that long chromo-
somes diminish fertility and viability. It is also more likely to be
of economic value for quantitative than subtle qualitative
characters.

A few examples will serve to illustrate these general points.
In the difficult genus Potentilla, numerous “species” are apomictic.
Some of these are allopolyploid (see below), otihers autopolyploid
(Miintzing, 1931). For instance, P. argentea (n = 7) exists in 2n,
6n, and 8n forms. Doubtless 4n types will also be discovered.
In general the h%h polyploids were more vigorous. One diploid
type was very small and prostrate, while one hexaploid, growing
only a dozen miles away, was tall and erect.

In the Central European crucifer Bismtella laevigata (Manton,
1934), the distribution of the diploid forms is restricted and
discontinuous, of the tetraploids continuous and much more
extensive. The diploids seem to be relict forms, confined to areas
which were not covered by the ice during the glacial period.

:3;38' B¥OiUTIOH:, THE MODERN SYNTHESIS.

Ob the other hand, most of the area now inhabited by the 411
ty^s was under ice during the periods of maxiiBum glaciation.
Thus we can safely^ conclnde that the tetraploid races were
evolved in response to the onset of colder conditions, and have
been, in virtue' of their ^eater vigour and cold-resistance, able
tO' colonhe large areas either unavailable per se to the diploids,
or where the diploids cannot compete successfully with the
tetrapioids to which they have given rise. An almost precisely
similar state of affairs is found in the North American genus
Ttadesemtia^ but here in several species (Anderson, 1937; and
see Dobriiansky, 1937, p* 196). In many cases, it seems clear
that the advantages enjoyed by autopoiyploids have enabled
them to supplant their opioid progenitors entirely.

Thus autopolyploidy, regarded from the evolutionary stand-
point, in general seems to provide a method by which a type
may become adapted to new and especially to less favourable
conditions. Once the polyploid forms have become established
and have undergone the necessary internal genetic adaptation
(p. 145) as well as further extern^ adaptation, they will often
extend their range far beyond the original diploid distribution,
and may frequently restrict the range of their diploid ancestors
through competition. In other cases the formation of an extended
autopolypioid series may enable a type to occupy a greater
variety of ecological niches. Crossing sometimes takes place
between members of a series, producing new polyploid types,
which then may be preadapted to still other conditions. Long-
term plasticity, however, is reduced by polyploidy (pp. 143,

374).

In one sense, the different members of close autopolypioid
series should be regarded as species, since they are kept quite
distinct by genetic barriers. The morphological differences between
them, however, are usually very slight, so that for taxonomic
.purposes' it is undesirable, to give them separate specific names,
and the totality of the forms may be named as one ^*polyploid
species'\ From what has been said above, it is probable that the
majority of 411 and higher autopolypioid forms of such polyploid
species are of geologically very recent origin. With the passage

SPECIATION, ECOLOGICAL AND GENETIC 339

of time, we may prophesy that the morphological differences
between them and their 2n ancestors will become more marked,
until they merit specific naming. It is presumably by such means
that some of the polyploid series of obviously good species in
various plant genera have been evolved. (See also pp. 347-8).

In general we may say that divergence based primarily on
genetic isolation has been of less evolutionary importance than
other types of divergence, its only major achievement being the
autopolyploid series of various plant groups. It has often been
of the greatest secondary importance, however. Once geo-
graphical or ecological isolation has separated groups, it is largely
die accident of genetic divergence, genic or chromosomal, which
eventually render the two types intersterile.*

9. CONVERGENT SPECIES-FORMATION

The most important type of diversification produced primarily
by genetic isolation is the origin of new true-breeding forms,

* A peculiar condidpn has recently been described in the wild millet Sorghum
purpureo^sericeum (Janaki-Ammal, 1940)* Is a diplpid (2n = 10), but

40 per cent of -w^d plants have from i to 6 (mostly 2 or i) extra so-called
“B-chromosomes” in their floral parts; these are, however, absent from the
roots.

The B-chromosomes do not pair at meiosis, and have a marked effect in
reducing pollen-fertility. To offset this reproductive disadvantage, there must
clearly be some considerable somatic advantage accruing from their presence in
the floral tissues. But how it operates, why they are absent in the roots, and
what the origin of the condition may have been— these points all remain
obscure.

Darlington and Upcott (19414), investigating a somewhat similar state of
aflain in maize, have come to some more generS conclusions as to the function
of these so-called inert or B-chromosomes. These, though variable in number,
exist with a definite mean size and frequency in various strains. Since various
agencies involved in the mechanics of mitosis and meiosis are constantly operating
to reduce both their size and the numbers present, some counter-selection must
operate in the opposite direction. Darlington and Upcott conclude that this
counter-selection is concerned with their special activities in nucleic acid meta-
bolism.

In the domestication of maize, the B-chromosomes appear to have taken
over and enlarged the metabolic function originally carried on by the hetero-
chromatic knobs which form part of some of the normal chromosomes. They
appear to provide a more elastic means of adjusting the plant’s nucleic acid
metabolism to the increased demands made on it by agriculturists in selecting
for higher yield.

Similar arguihents seem to apply in other cases, both in plants (e.g. Fritillaria,
Ranunculus, and Secale) and in animals (various Heteroptera),

as against secondary allopolyploidy. In addition, new forms are
here produced by convergence, not by divergence. Summaries
of the evolutionary effects of polyploidy are given by Darlii^ton
(1937) and Tischler {1941).

The classical case of species-formation by allopolyploidy is
that of Primula kewensis. This arose from a spontaneous cross at
Kew between two well-known species, P. vertidllata and P-
fioribunda, both with 211 == 18 chromosome. The hybrid was
originally entirely sterile. The chromosomes of the two parental
species were able to pair and segregate in spite of their dis-
similarity, but the resulting combinations of genes were so
abnormal that all the offspring were inviable.*

The hybrid was cultivated vegetatively for some years, until
a shoot appeared which was fertile. On cytological examination
this was shown to possess thirty-six chromosomes. The sterile
hybrid possessed one set from each parent— A'^, A^, B^, . . .

R’', R(, The fertile shoot possessed two sets: A'^A^, A^A^, . . .

R^^. Pairing could now occur between identical chromo-
somes. Every gamete thus possessed a complete set of chromo-
somes from both original parents, and viable offspring were
accordingly formed. Reproduction is not entirely normal, since

SPECIATION, ECOLOGICAI, AND GENETIC 34I

In this case, the sterile hybrid would probably not have main-
tained itself until the chromosome-doubMng occurred, without
human interference. However, that new species can be formed
by this method in nature is shown by the striking example of
the rice-^rass Spartina townsendii (Huskins, 1931). There seems
to be no doubt that this is an allopolyploid derived from the
crossing of the European S. stricta with die imported S. alterniflora.
The basic haploid number (x) of the genus is 7. S. stricta itself
appears to be an octoploid (an = 56) and S. alternijlora a deca-
ploid (an = 70). S. townsendii has an = ia6 == i8x. Most inter-
esting from the evolutionary standpoint is the fact that the new
species is in some as yet obscure way better equipped than eidxer
of its parents; it not only kills them out in competition, but is
extending its range beyond theirs. It is now being employed
by the Dutch for reclaiming land from the sea. This favourable
result of the interaction of two gene-complexes is the reverse of
that described on p. 66. It also demonstrates the role of range-
changes in this type of speciation (p. 348).

Two species of horse-chestnut are known to have originated
by hybridization. It is interesting that one of them is a parent
of the other. From the two tetraploid species, the European
Aesculus hippocastanum (tihe common horse-chesmut) and the
American A pavia (the red buck-eye) the pink-flowered octo-
ploid garden species A. cornea has arisen. This, on crossing with
A. hippocastanum, gave rise to A. plantierensis. In this latter case,
a hexaploid was produced, which at ont^ bred true without
further doubling. These and other examples are enumerated by
Darhnglon (1937, p. 234).

A very pretty example is the experimental synthesis of a wild
species ofhemp-netde, Galeopsis tetrahit. On various grounds this
tetraploid species was presumed to be the result of a cross between
G. puhescens and G. speciosa, both ordinary diploids. After crossing
these, an allotetraploid was produced, which is almost identical
with the wild form (Muntzing, 1932, 1937). Undoubtedly,
the wild species did originate from this cross, but has since
its origin undergone slight further differentiation by mutation and
selection.

342 evolution: the modeun syntiiesis

Occasionally what merits the title of a new true-breeding
specks is formed by hybridization itnthout subsequent chromo-
some-doubling. This may happen when, as in die Aesadus case
just mentioned, an isopolyploid is produced between two odicr
isopolyploids of different chromosome number. E.g. 4 x X 8x;
gametes 2x + 4x = 6x zygote. Otherwise, the process can only
occur when both parents have the same chromosome number,
and when the hybrid enjoys certam advantages over the parents.
If capable of sexual reproduction, the hybrid will of course be
exceedingly variable, as independent assortment between the

members of the tv'o parental genomes will occur.

The best case is that of the hybrid between two species of
Medicago, the imported purple-flowered lucerne (M. sativa), and
the yellow-flowered sickle medick (M. fakata) of Europe (see
Gilmour, 1932). The hybrid, originally described as a distmet
species under the name of M. sylvestris, has strange greenish-
black flowers, is exceedingly variable, and is both more vigorous
and more fertile than cither parent. In Britain this hybrid may be
dated back with reasonable certainty to the seventeenth century,
when lucerne was first introduced. One would coryecturc that
its initial variabiUty would have been somewhat reduced by
selection, but there is no direct evidence for this. In one region
of France, where lucerne has not been reintroduced for some
time, die hybrid appears to have ousted both parent forms
entirely.

Ledingham has recently shown (1940) that M. falcata exists
both in a diploid and tetraploid form (zn == 16 and 32), whfle
M. saUva always has 2n = 32, and is thus presumably tetraploid.
“M. sylvestris” involves the tetraploid form of M. falcata. Homo-
logous chromosomes of the two species pair and segregate freely
in the hybrid. Lcdingl^m wishes on this account to classify
M. falcata and M. sativa as “varieties of one higlily polymorphic
species”, but Mr. J. Gilmour assures me that no plant taxono-
mists hold this view. The two types have different distribution,
and differ in numerous characters, bodi morphological and
physiological. If one makes interfertility the sole criterion of
species, then the diploid M. falcata would have to be put

SPEGIATION, ECOlOGICAt AND GENETIC 343

in a distinct species from its almost identical tetraploid, while
this latter would be classed specifically with the quite distinct
M. satival

“M. sylvestris” is in one sense a hybrid swarm, on account of
its high segregating variability, but a hybrid swarm which is
capable of . permanent existence as a group-unit apart from
eiAer parent, and thus a new species, albeit one with peculiar
properties (see pp. 147, 355).

We may also cite the case of Phaseolus vulgaris and P. multi-
jiorus. These are two well-defined species, crosses between which
have recently been investigated by Lamprecht (1941). Both
species have n = 17 chromosomes. The cross only succeeds with
vulgaris as female parent, and the Fi is almost sterile. However,
by breeding from the few seeds produced, a number of constant
lines were obtained in F5~F9, some very close to one or other
pure parent species, others intermediate, showing all possible
combinations of the parents’ characters. Fertility was originally
low, but could gradually be raised to a high level. Here is an
excellent example of species-diiferences depending solely or
mainly on gene-differences. Further, the gene-complexes of the
two forms have gradually become largely, but not quite, incom-
patible, so that selection is 'still able to restore viability and
fertihty in the hybrid. Some of the hybrid lines can properly
be regarded as artificial species, since they are wholly or largely
sterile with pure P. ra/gan's.

Allopolyploidy has undoubtedly played an important role in
the evolution of many plant genera. The careful analysis that has
been made in Nicotiana will serve as a good example. Kostoff
(1938) has experimentally produced a new allopolyploid by
hybridizing N. glauca (n = 12) and JV. langsdorffii (n == 9). An
allotetraploid (n = 21) arose by parthogenesis. This showed
rather poor fertility, but fertility rose gradually in successive
generations, until it approximated to normal. The allotetraploid,
though possessing unique characters, was by no means constant,
throwing forms that difiered in numerous characters, both
morphological and physiological. This was due to the relative
frequency of heterogenetic pairing— i.e. pairing and consequent

EVOiUTlOH':' THB'- ^MODERN S;YNTM^

:ioii of cliromosomcs bcloiigiiig to ' the two . parciital
though hoiiiogenetic pairing was natorally the rule. Tliis
incy also decreased hi the course ol generations, but not
jrcat extent.

also produced an allotctraploid (211 — ^o) between
titmkm (n — 24) and N. stmuealens (11 == 16). In this
e new species was remarkably constant, owing to die
of hetcrogcnetic pairing.

her synthetic species, N, Jiglutd, was manufactured ten
)rcviously by Goodspeed and Clausen, by crossing AT.
(n = 24) with K glutinosa (n == 12); the hybrid became
>id with 72 as its somatic clironiosomc-uumber. Still

SPEGIATION, ECOiOGlCAL AND GENETIC 345

must be soi^ht in a form related to N. trigmophylla. Nothing
is known as to the reason for the widespread occurrence of
allopolyploidy in Nicoticma, but we may safely si^gest that the
opportunity for the necessary species-crosses was provided by
extensive range-changes in relation to alterations of climate and
land-level. ’

The wheats provide an equally striking example, with certain
different features (summarized by Dobzhansky, 1937, pp. 215
seq.). Wheats fall into three groups with n = 7, 14, and 21
respectively. Broadly speaking, the 21-diromosome forms
ivulgare group) have three distinct genomes, A, B, and D, of
which the 14-chromosome form (emmer group) have two (A
and B), and the 7-chromosome forms {einhorn group) the A-
genome only. Allopolyploidy appears to have occurred twice,
once with an unknown form providing the B-genome, and subse-
quendy with an Aegilops-like form introducing the D-genome.

Complications have been introduced by the genetic divergence
of various types. Thus one of the emmers, T. timopheevi, has a
B-genome which differs considerably from the normal. This
may mean that its B-ancestor was not identical with that of
other emmers, but a related species;* or possibly the differentia-
tion may have occurred subsequendy. Again, heterogenetic
pairing between members of the different genomes takes place
to a difierent extent in different cases. Obviously, a twice-
repeated allopolyploidy such as has here occurred provides the
opportunity for great diversification.

hi willows (Salix), Nilsson has been able to build up artificial
species of an amazingly synthetic nature. One artificial species
(Nilsson, 1936) contained genetic elements from no less than
eight wild forms. Similar cases are known in orchids. The hybrid
“genera” Potinara &nd Burrageara have been built up artificially
from four different species belonging to four distinct natural
genera (Sander, 1931). ,

Recently, the discovery that polyploidy may be artificially

* That allotetraploidy has occurted in more than one way is made probable
by recent Russian work; e.g. T. persicum (n x 28) seems to be an allotetraploid
derived from a cross between T. dicoaoides and Aegihps trimcialis, both with
n = 14 (seeWaddington, 1939, p. 323).

34*5 evolution; the modern synthesis

produced by colchicine has opened up new possibilities in this
field, since sterile hybrids can often be immediately converted
into more or less fertile allopolyploids by this means, instead
of waiting for the lucky chance of natural chromosome-doubling.
As an instance, we may take the recent work of Harland (1940)
on cotton. For example, he has synthesized an aHotetraploid
between the Old World Gossypium arboreum and the New World
G. thuTberi (both n = 13). This can then be crossed with com-
mercial tetraploid forms (which themselves appear to be the
product of allotetraploidy between Old and New World dip-
ploids), and the immunity to pink boUworm carried by G.
tkurberi can thus be introduced into cultivation.

Again, he has synthesized allohexaploid forms between the
commercial G. barbadense (n == 26) and various New W orld
diploids (n = 13). The addition of the wild genome confers
increased resistance to drought and to various pests, as well as
great vigour and sometimes high quality of lint.*

Allopolyploids, like autopolyploids, often differ in physio-
logical and ecological peculiarities from their diploid ancestors,
and therefore c6me to occupy different ranges; and, again as
with autoplyploids, their ranges are usually more extensive.
To take but one example, G. H. Shull (1937) has summarized
our knowledge of the species of the crucifer genus Capse/lff.
Here the basic chromosome-number (x) is 8. The diploid species
with 2n = 16 are, with few exceptions, found in the Mediter-
ranean area, which appears to be the original centre of distri-
bution of the genus. This region is also inhabited by various
tetraploid forms; but these, taken together, are world-wide in
their extension. Although Shull has not been able to detect any
diferences in vigour between the 2X and 4x forms, he is con-
vinced that some such differences must exist, together probably
with differences in adaptability, to account for the observed
distributional difference. While autopolyploidy may have
occurred, Shull is convinced tliat hybridization and allotetra-

* The use of cokhiciue has of course other appiication.lt permits the building
up of autopolyploids. These may themselves show valuable new characteristics,
or they may sometimes give fertile crosses with other polyploid species of the
same chromosome-numlSr, and so produce new recombinations.

SPECIATION, ECOLOGICAL AND GENETIC 347

ploidy has been the chief source of 4x forms. In one area of
Texas, two 4x forms, C. occidentalis and C. bursa-pastoris, which
are restricted to the region west and east of the Rockies respec-
tively, have met and crossed, producing a more or less stable
new type; ShuU does not give cytological data for this form.

The genus Potentilla is one in which both auto- and allopoly-
ploidy have occurred. We have already referred to the auto-
polyploid series in P. argentea (p. 337). In P. collina and P. aantzii
on the other hand, Miintzing (1931) finds strong evidence for
allotetraploidy, in the shape of anisoploid (5X and yx) forms,
Apomictic reproduction has enabled these heterozygous forms
to remain in permanency. Most collina hiotypes, however, are
hexaploid. Miintzing considers that the very variable “collective
species” P. collina arose through a cross between P. argentea and
a form close to P. tabernaemontani. In general, the species of
Potentilla “which are regarded as old and primitive ... are
characterized by low chromosome numbers and a relatively
limited and decreasing geographical distribution”, whereas the
dominant and aggressive types have a high chromosome number
(up to I2x), mainly due to allopolyploidy. See also Christoff

(1941).

A curious case is cited by Tischler {1941), where a gigas-
form of the normally hexaploid Aloe ciliaris turns out to be
pentaploid.

One iSnal case deserves to be mentioned, since it shows that
new species can arise by this means even after intergeneric
hybridization. This is the radish-cabbage hybrid Raphano-
Brfissica (Karpechenko, 1928). This is the product of a cross
between the radish (Raphanus sativus) and the cabbage (Brassica
oleracea), both with zn = 18 chromosomes. The 18-chromo-
some hybrid was at first sterile, as widi Primula ketoensis, but
became fully fertile on achieving allotetraploidy.

That allopolyploidy after species-hybridization has been an
important agency in evolution in giving rise to new species in
nature is shown by the large number of cases in which allied
species within a genus or group of genera show diromosome
numbers which are all multiples of some basic number. For

348 evolution: the modekn synthesis

instance, the basic haploid number (x) in the genus Ckrysm-
themum is 9, and 2 x, 4X, 6x, 8x, and lox species are knovm.
In wheat and oats x == 7, and 2x, 4x and 6x types occur. Similar
serira occur in every large genus of flowering plane as yet
investigated, with the exception of Rihes and Antirrhinum.

The evidence goes to show that while some of these series
may be due to tie occurrence of autopolyploidy (p. 335 )>
great mgority, notably of sexually reproducing types, are due
to initial allopolyploidy.

Kinsey (1936) suggests that species-hybridization (presumably
followed by allotetraploidy) has played a considerable role in
the evolution of the gall-wasp family Cympidae, but his con-
clusions are unsupported by experimental evidence. In general
it appears unlikely that this mode of speciation has occurred to
any extent in any animal group. It has been suggested that
polyploidy might be commoner aihong hermaphrodite animals
on account of the absence of the X-Y sex-determinmg mechan-
ism, but M. J. D. White (1940) has shoym that even here it is
much rarer than in plants. In moths, Federley has shown that
allotetraploid hybrids may arise through non-reduction of the
chromosomes in gametogenesis. But new species do not appear
to arise in this way, partly because mating preferences keep
normal species apart, partly because the hybrids are not fully
fertile (see Federley, 1932). Species-hybridization occurs in fish,
but we do not know the cytological phenomena. Hubbs (1940)
has described cases in which desiccation in the American desert
has brought tc^ether in one pool two species originally diferen-
tiated in relation to lake and stream life. E.g. with chub, two
pure species and many hybrids were foimd in a single section
of crccL Such cases would repay further investigation.

Considering the general role of allopolyploidy in plant evolu-
tion, we may conclude that it is likely to occur when changes of
chmatc bring about range-changes, these then providing oppor-
tunities for hybridization between plant species which have
developed in isolation and between which no reproductive
barriers have therefore been evolved, such as different flowering
seasons or adaptation to diferent insect pollinators. But, once

SPEGIATION, ECOLOGICAI, AND GENETIC 349

induced, allopolyploidy often provides the opportunity for taking
advantage of the new conditions. This is partly due, as with
autopolyploidy, to increased vigour and resistance, partly to the
fact that quite new types, some of which are pre-adapted to
various ecological conditions, are produced (pp. 336, 351), and
fipally to the plasticity conferred by a certain degree of hetero-
genetic chromosome-pairing.

This plasticity is due to the fact that heterogenctic chromosome-
pairing produces a unique type of variation. The mechanism of
meiosis produces segregation and recombination. The characters
segregated and recombined are in the vast majority of organisms
dependent on single-gene mutations w’hich form part of the
general constitution of the species. However, in allopolyploids
with some degree of heterogenetic pairing, what are segregated
and recombined are not single genes, but groups of genes which
have evolved for long periods, often millions of years, in isolation
from each other, so that such species possess a new kind of
recombinational variation in their genetic stock-in-trade.

We must finally consider the cases of so-called aneuploidy
or secondary polyploidy, in which some kinds of chromosomes
are represented more often than others in the total complement
(polysemy). Difierent strains ■within Viola kitaiheliana include
not only polyploids, and polyploids lacking one chromosome
(monosomies), but polyploids with some chromosomes poly-
somic (Clausen, 1927). The analysis here is not, however, so
clear-cut as in the species of dahlia, D. tnerckii. All species of the
genus Dahlia save this one have n = 8 or some multiple of 8;
D. merckii, however, has n = 18. C'ytological evidence proves
that this must be interpreted as a tetraploid in which two kinds
of chromosomes are represented by three pairs instead of two;
i.e. whereas most kinds of chromosomes will exist in the form
Fipi, F^F*; QiGh G^G®, two kinds will exist as A^Ah A^A^,
A®A 3 ; B^B^, B®B 3 . The species is thus mainly tetraploid,

but partly hexaploid. It is noteworthy that this species shows
more striking differences from the rest of the genus than does
any other. This is to be expected, since the balance between
the genes contained in diferent chromosomes is upset. Such

350

evolution: the modern synthesis

cases must have originated suddenly by duplication of whole
chromosomes, those (oims surviving which have a proj^r genettc
balance. They are not known for certainty in diploids: the
reason here is presumably that the upset of balmce wodd be
more considerable (2 ; i instead of 3 ■ 2 ,)* Collins, Holhng^
head and Avery (1929) produced a secondarily balanced^ speaes
of Crepis, C. artifcialis, by crossing the tetraploid C. btenms
(an = 4x = 40) and the diploid C. setosa (zn = 8). In the Fi,
the 20 biemis chromosomes formed 10 pairs by autosyndesi^,
but the 4 setcsa chromosomes segregated at random. However,
after some generations of selfing, a true-breeding stram w^
produced in which two of these had been completely lost, while
the other two had become paired. Here, then, the seconday
polyploid has been produced by loss of chromosomes. Accord-
ing to Sikka (1940) secondary polyploidy has played a con-
siderable role in the cabbages (Bmska). The basic chromosome
number is x = 5. Straightforward tetraploids (an = 20) eiit,
together with both plus and taittus secondary tetraploids (an
=20-1-4; 2n=20-2; 2n=20-4). From these various forms
allopolyploidy has produced new species, with zn = 34, 36, 38,

and 48 chromosomes respectively. r 1 1- ■ 1.

Darlington (1937) has given cogent reasons for believing that
the whole Pomoideae section of the Rose order, comprising the
apples, pears, medlars, etc., are of similar constitution, derived
from a basic number of x = 7, by tetraploidy foflowed by
extra representation (six times instead of four) of three chromo-
somes. They then have 2n = 34 > original seven

chromosomes being represented by two pairs each, the remain-
ing three by three pairs* It is probable, though by no means
certain, that this condition has been reached by the loss rather
than the addition of chromosomes after a cross. On paleonto-
logical grounds, this condition must have originated not later
than the early Tertiary- period.

* An anenploid form with extra representatioii of one kind of chromosome
lias been experimentally produced in tobacco (^NicotiaiM) by Webber (i 93 u) » the
result here followed five generations of in-breeding after a cross, and involved
various complex processes which need not concern us here. Lammarts (193 a)
has, by similar methods, produced another ‘‘species of this type.

SPECIATION, EGOXOGICAL AND' GENETIC . ■ 35I

Thus in addition to the various evolutionary implications of
polyploidy already mentioned', we see that it permits a new
type of initial variation, in the shape of alterations in the numerical
balance between different kinds of chromosomes.

10. HETICULATE DIFFESENTIATION

We must now briefly consider the extremely complicated state
of affairs to be found in certain plant groups like the roses (Rosa)^
brambles {Ruhm)^ willows {Salix)^ and hawthorns (Crataegus),
resulting in a network of forms (reticulate evolution).

In all these groups, matters are complicated by a combination
of polyploidy and various methods of non-scxual reproduction.
In the Caninae section of the genus Rosa, what has been called
subscxiial reproduction occurs. The species of this group typically
possess 35 chromosomes, 7 being the basic number for all roses.
In the formation of ova, 14 of these normally pair at meiosis,
wliilc the remaining 21 all go to one pole of the spindle. This
results in cells wdth 7 and with 28 cliromosomcs respectively,
and from the latter the ova are formed. In the formation of
pollen, on the other hand, no such differential behaviour of the
unpaired chromosomes is observed, but most of them arc elimin-
ated from the nuclei by lagging during division. The result is
that the majority of the viable pollen-grains have the complete
single set of 7, together with o, i, or 2 others. The 21 unpaired
chromosomes arc thus generally handed down ascxually, while
the few that appear in viable pollen grains provide a certain
amount of excess variability. The Caninae group has undoubtedly
arisen through hybridization — cither by repeated crosses between
different types of diploid species or by a cross between hcxaploid
species — after which the special peculiarities of the system must
have arisen adaptively. It is noteworthy that whereas self-
pollination leads to sexual reproduction, cross-pollination nor-
mally acts as a stimulus to parthcnogcnetic development.

In other sections of the gcinis, all the cvcn-multiplc polyploids
(and even certain of tlic diploids) appear to have arisen as species-

352 EVOLUTION':'^ THE M01>K8N' SYNTHESIS.

hybrids. In some eases these hybrids arc iptcrchangc hctcrozygotcs,
with the result that segregation produces forms resembling the
presumed original parents. Crosses between different species often
occur, and may be viable and capable of reproduction (Darling-
ton, 1937. pp- 460 scq.).

It will dius be seen that hybridization
the ^nus Rosa, and that as
breeding polyploid species,

either of sin gle cliromosomcs or genomes, takes place,
thus forms a network, in '

is not uncommon m
die result of it, in addition to truc-
a certain amount of segregation.
The group

wliic'h convergent species-formation
has not merely led to new species, but also to their partial or
total dissociation; and some of the new types produced by this
dissociation will maintain themselves. The course of events can
be represented as a network, so that we can speak of the evolution
of the group as reticulate (Turrill, 1936).

The same absence of sterility-barriers between related species
as is shown by Rosa occurs also in Rubus, though here the position
is complicated by the fact that whereas crosses between closely-
related species usually yield true hybrids, those between more
distantly related forms yield “false hybrids”. These are produced
entirely apomictically, aldiough the stimulus of the foreign
poUen appears to be necessary. A remarkable fact is that in New
Zealand, though reticulate evolution is frequent in the flora, it
docs not occur in Rubus (Allan, 1940).

Some hybrid Rubus forms breed true as new polyploid species
(this is also true of the loganberry, a species artificially produced
by allopolyploidy after a cross between raspberry and black-
berry). In nature, species occur with 2x, 3x, 4x, 5x, 6x,
and 8x chromosomes (x — 7, as in Rdsa), and apparently
divergent segregants as well as convergent hybrid forms are

SPECIATION, ECOLOGICAL AND GENETIC 353

breed true and maintain themselves in nature (and see p. 334).
Thus “many of the species and micro-species of Rubus are
evidently clones and subclones, produced by segregation and
maintained by apomixis”.

The willows {Salix) show the same bewildering multiplicity
of “species” in nature as do Rubus and Rosa, and almost certainly
for the same general reasons (see e.g. Nilsson, 1930), and synthetic
species have been artificially created (p, 345). Bewildering hybrid
swarms are found in New Zealand (e.g. in Aleuosmia, etc. p. 355;
Allan, 1940). hut cytogenetic investigation is needed before we
can say if they are of the true reticulate type, or merely show
mendelian gene-recombination. Similar but less extreme “reticu-
lation” appears to occur in one section of the genus Viola.

As a result of these processes, the classification of such groups
according to ordinary criteria is rendered all but impossible.
We may quote what an experienced plant systematist has to say
on the matter (Turrill, 1936): “The taxonomy of the British
genus Rubus is in such a state that specialists sometimes cannot
agree in more than one determination in ten. It is probable that
in such genera a totally different scheme from tliat of species
and varieties will have to be evolved before stability of expression
is reached.”

The case of the hawkweeds {Crepis) is dealt with elsewhere
(p. 372). Babcock and Stebbins (see Stcbbins, I940fl) propose the
term polyploid complex for groups in which self-perpetuating
secondary hybrids between auto- and allopolyploid forms are
produced, so that “there arises a complex network of interrelated
forms, which defies classification according to the usual concepts
of the species” — ^i.e. which shows reticulate evolution. Reticulate
polyploid complexes of this sort occur not only m Crepis, Rosa
and Rdus, but in scores of other plant genera.

This “convergent-divergent” type of reticulate evolution may
be contrasted with the “recombinational” type found in man.
Here, a reticulate result has been achieved by quite other means.
Instead of the initial crossing being between distinct species, and
the divergent variabihty being due to segregation of whole
chromosomes or genomes, the crossmg appears to have taken

place between, wel-marked* geograpnicai suDspeacs,

divergent variability is thus due to ordinary gene recoinbmation.
So far as we know, no polyploidy and no formation of spemiiy
stable types has occurred, but the progressive increase of migra-
tion and CTossing has led to a progressive inc^ase of general
variability (sec general discussion in Huxley and Haddon, 1935 >

^Mm 'is die^ only organism to have exploited this method of
evolution and variation to an extreme degree, so that a new
dominant type in evolution lias come to be represented by a
single world-wide species instead of showing an adaptive radia-
tion into many intcrstcrile species. Doubtless this is due to his
great tendency to individual, group, and mass migration of an
irregular nature, coupled with his mental adaptability which
enables him to effect cross-mating quite readily in face of differ-
ences of colour, appearance, and behaviour which would act as
efficient barriers in the case of more instinctive organisms.

Keith and McCown (1937) refer to the extraordinary varia-
bility of Palestine man some sixty thousand years ago. As a
“neandcrdialoid type can be distinguished in this population, it
may be suggested that the variability is pardy dependent on
crosses with H. neanderthalensis.

It is interesting that in the animal group with the largest
powers of irregSar aspersal, the birds, adumbrations of Ae
same process occur. We have already spoken of the hybridization
of the two species of flickers (p. 250), but in addition to the
“mixed zone” where the two species have come into contact
by extending their ranges, obviously hybrid birds are found
sporadically in the areas of the pure species, the frequency of
such forms naturally diminishing with distance from the mixed
zone (Taverner, 1934)-

We have also (p. 252) mentioned the somewhat similar picture

* Some authors, such as Gates (1930), prefer to call them distinct species:
the dilfcrcnce is here largely a matter of convemciice, but since they are clearly
of geographical origin and aimpletcly or reasonably interfertile, so tiiat the
resultant largely hybrid group constitutes a single interbreeding unit, it scenu
better, and more in accord with modern practice, to style them subspecies ot
a single large species or Rassenkreis (p. 1 ^ 3 )*

S]PECIATION, ECOLOGICAL AND GENETIC 355

presented by the red-tailed hawks of the genus Buteo in Canada.
Here the number of sporadic hybrids occurring within the areas
of the various normally pure types appears to be more consider-
able (Taverner, 1936). Taverner points out that the occurrence
of sporadic individuals of the general type of one subspecies or
species within the range of another occurs in a fair number
ofbirds.

A slightly different effect is shown by the water-thrush (Seiurus
mveboracensis), z migratory species. McCabe and Miller (1933)
fintl that this species shows “incipient geographic differentiation’*
into three statistically separable subspecies, but “even in the
geographic centres of one of these races individuals may appear
that show a considerable approach to the other race. Geographic
segregation and correlation of characters ... are incomplete
not only at the borders or zones of intergradation, but to some
extent throughout each race”.

We have already referred to the hybrid swarms produced by the
crossing of plant species in nature (pp. 147, 353). These may be so
extensive and so successful that they constitute a definite element
in the flora of a country. Evolution in such cases is also reticulate,
though the meshes of the biological network will not be so
large as in Rubus or Rosa, and the result is more like that obtaining
in man. The best-investigated cases come from New Zealand,
where no fewer than 491 hybrid groups , have been recorded
(Allan, 1940). AUan refers to “colonies of Hebe that present a
multitude of forms none of which can at present be separated
out as belonging to a ‘good species’ ^e same sort of thing
occurs in Leptospermum and Senecio. In Aleuosmia there is an
extraordinary multiplicity of forms, many of them hybrids, in
the northern part of North Island, N.Z. ; the complexity diminishes
with increasing latitude, until in the southern part of the island
only a single well-characterized species is found.* As Allan says,
new methods of nomenclature must, be devised to deal with
such situations.

In many cases the hybrid swarm arises as a result of human

* We have here a very tumsml form of dhie-4ii degree of interspecific
crossing.

356 evolution;- .'THE ;:MOI>BRN, SYNTHESIS ,

interference (see p. 258). This probably appHes to the kidney
vetch, AnthyUis mlneraria (Marsden-Jones and TurnU, I 933 j.
where what seem originally to have been well-marked geogra-
phical varieties (subspecies) have hybridized in numerous areas
to give rise to complex hybrid swarms, each with its own

characteristics (sec also pp. 247. 291)- .

It has been suggested (Lowe, 1936) that the state of affairs to
be found in the ground-finches of the Galapagos and the sickle-
bills of Hawaii is to be explained as the result of large-s(^e
crossinc and reticulation. However, we have seen (p. 32?) that
this is not supported by more careful analysis. Hybn<^tion
does seem to occur occasionally, however. Lack (i 940 fl) men-
tions Geospiza cimrostris darwim,vMch. occurs on a smgle island
and appears to have arisen as a hybrid between G. c. propmqm
and G. magmostris: as would be expected it is exceetogly
variable. One other such case is also known in the Geospizids.

It will be seen that reticulate evolution, though uncommon,
is not so uncommon as was until very recently supposed. There
is a natural reluctance among systematists to recognize its e»s-
tence and its implications, since these run counter to the gener%-
accepted basis of taxonomic practice. The fact that this basis is
largely unconscious merely enhances the reluctance to change
It may be that once the necessity of admitting the existence of
reticulate differentiation has been recognized in principle, it wul
be detected, in large or small degree, in a much greater nimber
of instances, especially among plants, but also among animals.
In the latter case, it is likely to be of the smaU-meshed or recom-
binational type only, while in plants both this type and that of
the polyploid complex are to be expected.

To illustrate how different the methods of speciation may be
in higher animals and higher plants, two concrete examples are
here presented of genera which have recently been subjected to
the fullest analysis— the fruitflies {Drosophila) and the hawkweeds
(Crepis).

SPECIATION, ECOLOGICAL AND GENETIC 357

Drosophila is unique in being the only genus among either
plants or animals in which we have at our disposal not only the
results of intensive taxonomic study of the usual type and of
work on ecology and population-structure, but of an astonish-
ingly complete genetic analysis and of what we may call ultra-
cytology (of salivary gland chromosomes). Furthermore, it has
a very wide range, and comprises a large number of species. Ih
what follows, the accounts of Muller (1940) and Spencer (1940)
have been mainly drawn upon, while Dobzhansky’s book (1937)
has also proved a mine of information, and M. J. D. White
(1937) gives a brief but useful summary.

A recent careful taxonomic study by Sturtevant (1939) has
been imdertaken in forty-two species of the genus available for
detailed examination. Twenty-seven characters were selected
which could not be regarded as due either to similarity of
developmental processes, or to selective (adaptive) agencies, and
their correlations tabulated. Further work is in progress, but on
the basis of the results to date it was found that the accepted
taxonomy should be modified, and the genus should be divided
into three subgenera, one so far containing only a single species
(D. duncani), one (subgenus Drosophila) containing such species
as funebris, hydei, repleta, and virilis, and the third (subgenus
Sophophora) containing other weU-analysed species such as
melanogaster, sitmlans, athabasca, azteca, ananassae, mircmda, obscura,
pseudoobscura, and subobscura.

The two main subgenera differ in such points as the fusion
or separation of the posterior pair of Malpighian tubes, the
number of filaments on the egg, and the shape of the dark
posterior bands on the abdominal segments.

A considerable time ago Sturtevant in his monograph (1921)
showed that many of the specific characters in the genus could
be matched among the mutant characters which appeared in
experimental cultures.

The conclusions reached on the basis of cytogenetic work are
as follows. Firstly, Drosophila is as yet the only organism in
which sufficiendy detailed tests can be carried out to decide
whether, in addition to obviously mendelizing charaaen, the

358 evoiution: the modern synthesis

vague types of variation, more fluid and more continuom in
their phenotypic variation and their inheritance, also mende ze
and are therefore dependent on chromosomal and peculate
inheritance, or are due to some quite different type of process
(cytoplasmic inheritance, organismal relations, etc.). The answer
is decisive; with one possible exception, perhaps due to a virus
(L’HMtier and Teissier, 1938), all heritable differences in Droso-
phila are chromosomal. The further important conclusion can
be drawn that “small” mutations, with sHght effects, many ot
thftn often affecting the same character, are more frequent than
large ones, and much more important in evolution (Muller, 1940).

Another important general result (p. 755 Dobzhansky, l 939 o)
is that wild populations are full of gene-differences, mosdy
recessives in single dose, to an extent much greater than originaMy
thought possible. These gene-difFerences must be presumed to
have originated by mutation of the same type as has been studied
in the laboratory, though recent work (p. 55 > Zuitin, I 94 ^)
indicates that, owing to the rapid changes in temperature, etc,
in the wild, the mutation rate in nature may be considerably
higher than in standard laboratory conditions.

Detailed population analysis has also revealed the important
fact that populations from different areas, though superficially
alike, often differ in regard to their content of recessive genes
and also of chromosome rearrangements. Further, different species
of the genus show different degrees of this local differentiation,
doubtless owing to differences in behaviour and ecology (see
Dobzhansky, 1939&, N.W. and E. A. Timofeeff-Ressovsky, 1940).

Quite recently Spencer (cited by Muller, 1940) has shown
that a limited fertility exists in crosses between two species of
the genus— D. virtUs md D. amerkam^ The genetic analysis
thus made possible revealed that all the character-differences
investigated were due to multiple genes, each having a small
effect.

* The latter is sometimes referred to as a subspecies of D. virilism but &e
differences between the two in ordinary taxonomic characters, in polyploidy,
and-in chromosome morphology are, according to Spencer, as great as those
l^stween various pairs of fbnns within the genus, which are universally recog-
nized as ‘‘good species”: but see below, p. 367.

SPECIATION, ECOLOGICAI. AND GENETIC 359

Althougli the hybrids between D. mehmgaster and D. simulans
are wholly sterile, Muller, by an ingenious method (Muller and
Pontecorvo, 1940), has recently been able to obtain flies with
combinations of the chromosomes of the two species which are
equivalent to the -results of a back-cross between a hybrid Fr
and the melanogaster parent. The results show that in both species
ea ch major chromosome (X, II and III) contains interacting genes
affecting viability and another interacting system afiecting fer-
tihty. The small IVth chromosome o£ simulans, when transferred
into an otherwise pure melanogaster genotype, produces various
new genetic effects. Again, the abnormality of abdominal band-
ing and bristles shown in the normal Fi turns out to be due to
interaction between a sex-linked simulans gene and one or more
autosomal melanogaster genes, a result reminding us of the
melanotic tumours produced by species-hybridization in certain
cyprinodont fish (p. 66).

In general the results thus support the view that once groups
become isolated they start to diverge in respect of a number of
genes, and that these are interlocked in a harmoniously-stabilized
system or systems. After a certain time, apparently not of great
duration, the specific systems become mutually inharmonious or
even incompatible through the sheer accumulation of difference.

It has sometimes been suggested that sterility between species
depends on special factors. Here, again, the evidence from
Drosophila is to the contrary. The two “races” (well-differentiated
genetic subspecies) of D. pseudoohscura show a marked lowering
of fertility on crossing, die Fi males being wholly sterile, the
females very sHghtly fertile. By an ingenious method (only
possible in a genetically well-analysed form), Dobzhansky has
shown that, in later generations from back-crosses of the hybrid
females to either pure species, the fertility of the males depends
wholly on the particular combination of “fertility genes” (or,
if the term be preferred, “steriMty genes”) which they happen
to receive. These genes are distributed through all the chromo-
somes, and furthermore (as was also noted for the uirilis-americana
cross) different strains of each parent type differ in their cross-
ability, owing to their containing different complements of

360. : evolution: -THE., MODERN ; SYNTHESIS. ,

fertility genes. Thus the special criterion of most animal species,
their mutual infertility, appears in Drosophila to depend largely
on gene-mutations (together with sectional chroniosomal muta-
tions, as mentioned later).

In additioB, the intensive genetic work .carried out on single
species of the genus, notably D. melanogaster, has shown that
they al contain the genetic potentiality for developing strains ^
with reduced interfertility if evolutionary occasion should offer.
The most interestin,g and relevant cases are those in which a
combination of genes exerts an eiSect on fertility which is not
exerted by any of the genes singly. Thus mrly wing and moire eye :
when ill combination give males with almost complete infertility.
An' opposite effect is found in relation to deltex^ which in most
stocks thickens the wing-veins and also produces complete male
sterility. However, Bridges has found three separate ‘‘deltex-
suppressor’’ genes, two autosomal and one sex-linked, which
almost wholly suppress both the morphological and the sterility
effects of deltex.

Similar effects are known for viability (see Chapter 3). Muller
points out that such effects, of reduced fertility and viability
after crossing, are bound to arise sooner or later in strains that
are in any way reproductively isolated from each other. "Tor,
given enough mutational differences, some at least of the genes,
in recombination, will give non-additive effects on viability or
fertility, and, as is always the case with effects not yet subjected
to the sieve of selection, these effects wiE far oftener be adverse
than beneficial'' Thus any partial genetic isolation wil auto-
matically tend to become more complete with time. Further-
more, natural selection will also operate to reduce the wastage
caused by any degree of lowered fertility after crossing. Fi
sterility wiE, be favoured as, against Fa sterility, ' Fi 'inviabEity
as against Fi sterility, and mutations preventing Pi crossing at
aU (psychological and reproduaive barriers) as against those
concerned with effects on Fi.

The same general ' principles apply to viability, to normality
of development, and to normality of chromosome-conjugation
in meiosis. GeneticaEy isolated groups are bound to develop

SPBCIATION, ECOtOGIGAI. AND GENETIC 361

their own characteristic systems of genes adapted to harmonious
development and function, but these gene-systems are equally
bound to become more or less disharmonious vis-a-vis each
other, so that crossing will produce some reduction of har-
monious functioning in Fi or later generations. In particular,
certain genes which on their first incorporation were merely
advantageous deviations will become converted into necessary
bases for later stages of the genetic system.

It is extremely unlikely that hybrid sterility in higher animals
Csa ever be brought about by a single gene-mutation or a single
sectional rearrangement. On the other hand, reduction of cross-
ability might be brought about in this way, and would then
lay the foundation for the development of intersterihty.

The existence of a highly differentiated sex-determining
mechanism, as in Drosophila, provides an extra cause of hybrid
sterility. In the first place, the X-chromosome (in Drosophila)
contains a disproportionately large number of fertility genes;
and secondly, sex-linked genes must have especially strong
expression, since in the heterogametic sex one dose of these
genes must be balanced against a double dose of any comple-
mentary autosomal genes. Thus in crosses between incipient or
full species, sex-linked genes in the male Fi are especially likely
to be in imbalance with their autosomal complements, resulting
in lowered fertility of this sex. This is the basis for Haldane's
rule of the reduced Fi fertility of the heterogametic sex in wide
crosses; as Muller points out, similar or even larger efiects may
be exerted in later generations. Here again the species of higher
animals may be regarded as being more highly difierentiated,
and more sharply delimited genetically, than those of plants.

Drosophila, owing to its giant salivary gland chromosomes, is
especially favourable material for studying sectional chromosome-
rearrangements. Recent work has made it possible to evaluate
the degree of their importance in speciation with some degree of
assurance. The first point to notice is that sectional rearrangements
tend to impede crossing-over. Wherever crossing-over is thus
interfered with, and the sectional rearrangement is fairly wide-
spread within the species, the genes in the rearranged section arc

: -'M*. :■

362 V ."evolution: modern synthesis

cfFectively'isolated.from those in the corresponding ‘normaf’
section. Thus, as Darlington especially has stressed, ^ there are
produced within the species two isolated partial genetic systems
■which may diverge from each other like two distinct species,
by the accumulation of difierent mutants (see p. 33^5 also ,67? 139)’

It was at first thought that large rearrangements were common
■causes of spedation in Drosophila, but MuEer points out that
.in this regard they must be quite secondary to gene-mutation.
This is shown by the fact that the types of rearrangement which
most commonly characterize related species also exist commonly
within species.

In any case, they are much rarer than gene-muta-tions, and
can hardly ever recur identicaUy, as happens with numerous of
these latter. Furthermore, the above-noted fact of the genetic
isolation of the rearranged section from its normal homologue
wiE mean that, so long as the rearrangement remains rare, it
wiE not have the same evolutionary piasticit7, so that its possessors
wiE be handicapped if adaptive change is demanded. Thus
rearrangements are only likely to become estabEshed through
the accidental process that Sewall Wright calls ‘"drift”, which
wEl be favoured by the existence of small more or less isolated
populations. Their maintenance may also be favoured by their
heterosis effect, in the heterozygous condition, on vigour and
productivity.

Finally, individuals heterozygous for sectional rearrangements
are destined, owing to their peculiar behaviour at meiosis, to
give rise to a certain proportion of gametes with an unbalanced
gene-complement (included by Muller under the term aneuphiijf
w'hich give rise to inviable offspring. The resultant reduction of
productivity is especiaEy marked with translocations, increasing
with the size of the translocated section; within tliis type of
rearrangement, mutual translocations of practicaEy entire chromo-
some-arms suffer least. It also occurs with pericentrie inver
which include regions on either side of the centromere, and then
is more or less proportional to the length of the inversion.

On the other hand, paracentric inversions, which do not include
the centromere and lie wholly within one chromosome-arm.

SPECIATION, ECOLOGTCAt AND GENETIC 363

do not suffer this reduction of productivity. Sturtevant and
Beadle (1938) have shown that this depends on the fact that in
Drosophila the polar bodies are all formed in a straight line
extending radially outwards, and that this causes the aneupioid
chromosome-sets to remain in the two central polar bodies, the
egg nucleus and the outer polar body receiving normal gene-
complements.

Small “repeats” (including “shifts” of the repeated section
into another part of the same individual chromosome) will not
cause any serious loss of productivity.

The changes that lead to the resolution of a V-shaped chromo-
some into two rods, or to the reverse process, also occasion little
or no loss of productivity. It was originally supposed that such
alterations were readily brought about, but a proper under-
standing of the chromosomal mechanism has shown that they
require a combination of several relatively rare events, and must
themselves thus be much rarer than ordinary sectional rearrange-
ments (pp. 365-6).

The analysis of sectional rearrangements as fomid in nature,
both within species and as characteristics of closely allied species,
confirms the expectations derived from what we have just set
forth. Li many thousands of chromosomes from wild popula-
tions of several Drosophila species Dubinin and his associates
(1934, 1936) found only thirty-five sectional rearrangements.
Thirty-three of these were paracentric inversions (many of them
widespread), one was a small shift, and one a small translocation.
This emphasizes both die difficulty of other types of rearrange-
ment becoming established, and the rarity of rearrangements in
nature as compared with gene-mutations.

Again, D. simulans and D. melanogaster differ sectionally. in
respect of one large and one very small inversion; but they are
characterized by a large number of visible character-differences,
which must be ascribed to gene-mutations. A large number of
gene-differences have been shown to exist between D. virilis
and D. americana, while sectionally they differ only in two
inversions and probably one shift.

A very interesting point, however, is that in some species of

MODERN SYNTHESIS

the geBDs sectional rearrangements' are imich more niimeroiis.
This - is so - in D. pseudoobscura, which has yielded twenty-five
sectional diiferences (almost all intra-arm inversions), as against
seven discovered in ffiel^ftogastcT, In this case, different com-
bmations of sectional differences are found in different regional
populations, and each such population shows the same degree
of prevalence of sectional rearrangeincnt as characterizes the
entire population of species such as D. melanagaster. The local
groups also differ in regard to numerous gene-mutations. It is
thus probable that D. pseudoobscura differs biologically from other
species of the genus in being split up, owing to some ecological
peculiarity, into relatively isolated local groups, which wil
facilitate die local accumulation of sectional rearrangements
through SewaU Wright's '‘drift". The same cause has doubtless
operated to divide it into the two more or less intersterile sub-
species, “races" A and B. D. montium is also divisible into similar
“races"— at least two and probably more. In its race B one arm ot
one of the V-shaped large chromosomes is absent; possibly it is
genetically inert in the other race (Kikkawa, 1936).

So far, only very closely-related species have been considered.
When a greater degree of divergence has occurred, many more
sectional rearrangements have accumulated. Thus, although D.
pseudoobscum is quite closely related to D. mirmda^ Dobzhansky
and Tan (1936) have shown that at least forty-nine chromosome-
breaks, and probably more like one hundred, must have occurred
in the course of their differentiation from a common ancestor.
Patterson and Crow (1940) point out that the small size of the
breeding-units of D, miranda would allow a large number of
rearrangements to become irreversibly fixed, whereas the larger
and less isolated groups of D. pseudoobscura will promote a smaller
number of rearrangements floating through the population, ani
fluctuating in frequency. D. athahasca and D. azteca differ in a
still greater number of rearrangements, and, though they are
not widely remote systematically from D, pseudoobscura and
D. miranda, show no recognizable homology with either of these
in the banding of their salivary gland chromosomes. It is thus
probable that all these four species share the biological peculiarities

SPEGIATION, ECOLOGICAL AND GENETIC 365

ofD. pseudoobscura, whereby sectional rearrangements are accumu-
lated with greater frequency than in forms like D. melanogaster.
There is again no recogniaable similarity in chromosome-banding
between D. mdanogaster and D. pseudoobscura.

One interesting bearing of sectional rearrangements on taxo-
nomy results from their relative rarity, and from the fact that
most of those occurring in a single chromosome-arm, especially
if they involve sections of the chromosome-map which overlap,
cannot undergo recombination with each other by crossing-
over. It is accordingly possible in certain cases to deduce with
certainty or high probability the phylogenetic course of events
by which two related species showing a number of differences
m sectional arrangements diverged. The only restriction on the
method is that the seriation of steps can be read in either direc-
tion: to decide which is the origin and which the terminus, we
must rely on other data, such as morphological resemblances
and geographical distribution.

As an example we may take the rearrangements found in the
third chromosome of D. pseudoobscura. It is found that all the
rearrangements of race A must have a common origin, and so
must all those of race B. These two original types are both
removed by one step only from a configuration which no longer
appears to exist, but which was presumably ancestral. In addi-
tion, from this presumed ancestral type, the rearrangements
found in the related D. miranda can also be derived. Correspond-
ing studies on other chromosomes, together with data on the
geographical distribution of the various rearrangements, should
add considerably to the accuracy of the results (Sturtevant and
Dobzhansky, I93<5; and see Muller, 1940, p. 333).

It is well known that the different species of Drosophila differ
in the gross morphology of their chromosomes. Thus the haploid
melanogaster has 2 1 rod (X), and i dot (mierpehromosome) ;

willistoni has no microchromosome, 2 V’s and i rod, but the X
is here a V; virilis lias 5 rods and a microchromosomes; immigrans
I V and 3 rods; etc.

It was at one time thought that this would throw hght on the
taxonomic relationships of the genus. Later research, however,

366 evolution: THE MODERN SYNTHESIS

has made it clear that this is not so. As Muller (i94o) puts it,
“Evidently the species wander back and forth between one
metaphase picture and another, so that quite closely related
species may show very different chromosomal pictures. Even
such close forms as D. virilis and D. americana differ in such an
important respect as whether the X is a V or a rod.’

The chief processes at work in changing the metaphase picture
are:' (i) fusion or separation of whole arms — ^umon of two rods
to form a V ot vice versa. This is more readily accomplished
between autosomes than between the X and an autosome.
{2) The acquisition or loss of microchromosomes (dots). This
may occur comparatively readily because of certain technical
reasons, whereas the formation or loss of a new chromosome of
considerable dimensions would be impossible. (3) Marked
change in the size of a given arm; This is the least frequent of
the three. All these changes afe likely to be much rarer than
ordinary rearrangements, but not so rare as not to occur and
become established with some frequency when geological time
is considered.'

Certam of the changes have consequential genetic effects.
Thus when an X-rod becomes attached to a previously auto-
somal rod, the dosage relations of the genes in the new X-system
must undergo alteration, implying a modification of the whole
gene-complex. Incorporation of autosomal material in the Y
win lead to its gradual genetic degeneration to the status of
inert material.

Recently Stone and Griffen (1940) have experimentally altered
the chromosome pattern in D. melanogaster by translocation. In
one stock, the chromosome-number was reduced. The dot-like
IVth chromosome was translocated (possibly apart from a very
smaU residual portion) to the X-chromosome and thus became
hemizygous in the male sex. Three different sub-types have
been produced! In one, IV was transferred to one end of die X,
resulting in a J-shaped chromosome, in another to its other endj
and in a thkd it was inserted into the body of the X.

In another stock, an additional small chromosomc-pair was
produced by an elaborate process, resulting in part of the X-

SPECIATION, ECOtOGICAL AND GENETIC 367

chromosome now being represented in excess (sectional hyper-
ploidy) as a new small autosome.

These and other conversions were all at a disadvantage against
the normal as regards viabihty, but the disadvantage was not
very great and Stone and GrifFen anticipate that full viability
may be fairly soon restored by selection for modifiers, mutational
or recombinational.

Other recent studies from the University of Texas are of
interest in throwing hght on the various modes of speciation
in the genus.

Patterson, Stone, and Grifien (1940) have studied the forms
allocated to D. virilis. The species falls into two groups, (a) forms
with red pupae, pupating at die edge of the food, and with
adults highly susceptible to ether, {b) forms with grey pupae,
pupating on the side of the container above the food, and with
adults more resistant to ether. The “red” group includes two
subspecies, D. v. americana (regarded as a full species by Spencer:
see p. 358), and D. v. texana; the “grey” group includes but one
subspecies, D. v. virilis, but this shows some differentiation even
witHn.the U.S.A., and its Asiatic form is also somewhat distinct.
The species is rare and local in America, but abundant in eastern
Asia. D. virilis is unusual in showing marked chromosomal
differences between its subspecies. D. v. virilis possesses 5 rods
and a dot as its haploid complement; in D. v. texana Nos. 3
and 4 ofD. v. virilis are fused to form a V; and D. v. americana
has Nos. 2 and 3 and also X and 4 fused to form 2 V’s (in the
female; in the male there is no Y-4 fusion). There are also some
inversions as between the different subspecies. But the main
causes of isolation between the forms are sexual (behaviour)
isolation, low viabihty of Fi eggs, and compheated fertdity
relations. Thus in male hybrids between “red” and “grpy” forms,
those containing a “red” Y-chromosome must also contain 2nd
and 5th chromosomes from the same parent strain if it is to be
fertile. This relationship causes high sterihty in the subspecific
crosses that go easily, namely with “red” males, whereas the
reciprocal cross, though it can only be made with difficulty, is
fertile. This apphes only when “grey” forms from northern

368 EvotaTioN ; the modern synthesis

U.S.A. are used; die south-western and Asiatic types will not
cross at all v^ith “red" forms.

The net effect is that in some regions a certain amount of
gene-transfer is likely to occur between the two subspecies, or,
as we had better call them, semispecies. And this, as Sewall
Wright has shown for ordinary subspecies (p. 229), will be
beneficial in conferring greater plasticity in evolution, though
the loss of productivity due to crossing will act as an immediate
offset against this long-term advantage. A curious fact is the high
degree of sterility found in pure cultures of both “red” subspecies.

The home of the species (or supraspecies) appears to be
eastern Asia, and die forms of the “red” group we may con-
jecture have differentiated owing to “drift” in the sparse popu-
lations found in less favourable areas.

The case of D. mulleri and its relations (Patterson and Crow,
1940) is equally interesting, but quite different.

The group consists of D. mulleri, with the two subspecies
D. m. mulleri from Mexico and Texas and D. m. mojavertsis, a
pale desert form from the desert area of California, and D.
aldrichi, also from Texas (but probably from Mexico as well),
but with a rather more restricted distribution than D. m. mulleri.

The mulleri-aldrichi pair are very similar to the melanogaster-
simulans pair, in resembling each other closely (there are a few
minor but diagnostic character-differences), and in overlapping
considerably in their distribution. In both cases there is com-
plete genetic isolation between the members of the pairs in
nature, but the isolation has proceeded a stage further in the
mulleri group, since the cross between the two can only be made
one way. Ecobiotically there is more differentiation, aldrichi
taking considerably longer for its development than mulleri.
A few Fi male hybrids (all hybrids are sterile) have been dis-
covered in nature, showing that some reproductive waste still
occurs. The gross chromosome-structure is similar and there are
few if any large sectional rearrangements. It would appear that
the ancestor of D. aldrichi developed genetic incompatibihty
with D. mulleri while isolated, that the incompatibility was due
in the first instance to die progressive accumulation of gene-

SPEGIATION, ECOLOGICAI. AND GENETIC 369

mutations (though it may luve been strengthened later by
selection: see p. 287), and that, once present, it permitted aUrkhi
to spread and to exist side by side with mulleri.

D. m. mojavensis appears to be a true geographical subspecies.
The distributional centre of the species appears to be Mexico,
and it is a warm-climate form unable to tolerate low winter
temperatures. Its spread northwards into U.S.A. was thus restricted
to the warm plains on either side of the Mexican-Rockies moun-
tain system, and the western group, reaching the Californian
desert, there evolved into a markedly distinct subspecies, D. m.
mojavensis. This shows several large sectional rearrangements
which were able to establish themselves owing to isolation;
but it still produces fertile offspring with D. m. mulleri, at least
in one of the reciprocal crosses.

Patterson and Crow compare the mulleri to the pseudoobscura
group (D. pseudoobscura A and B and D. miranda). In each case
there are three forms, one of which behaves as a good species
while the other two are best regarded as highly difierentiated
subspecies. There are some differences. Thus the miranda-pseudo-
obscura cross is almost but not quite sterile. Further, the ranges
of pseudoobscura A and B overlap, and probably stiU exchange
genes in nature; and the visible ecoclimatic differentiation of
D. m. mojavensis (pale colour) is not found in D. pseudoobscura.
The chief difference is in regard to sectional rearrangements, of
which there are many between pseudoobscura A and B and still
more between either of these and miranda. This appears to be
correlated in part with the abundance of the mulleri forms and
the greater size of their breeding populations. Patterson and
Crow suggest further that sectional rearrangement in the mulleri
forms may be accompanied, as in D. melanogaster, by breakage
effects, in the shape of visible and lethal pseudo-mutations.
This would tend to keep rearrangements down to a minimum.
Such effects must be neghgible or absent in the pseudoobscura
group. These examples illustrate vividly the unexpected modes
of taxonomic differentiation to be found in insects.

Interspecific grafting (Stubbe and Vogt, i94oi) has revealed
that diferent Drosophila species differ both quantitatively and

jyo evolution: THE mobeen synthesis : ■ : /

qualitatively in regard to the precursor substances involved in
eye-colour differentiation.

An' interesting point for which no adequate explanation has
yet been found is thatj whereas in most Drosophila species, many
clear-cut characters with sharp dominance are found, in D.
pirilis most characters are determined by multiple factors, often
.with incomplete dominance. It was a lucky chancc that D.
melanogaster and not this species was first chosen for genetic work.

Muller is careful to point out that other groups of animals
(let alone plants) may have genetic mechanisms which do not
favour the same kinds of evolutionary change as in Drosophila.
We have already mentioned the fact that having the polar bodies
all in one line, while no crossing-over occurs in the male, permits
Drosophila to accumulate intra-arm inversions with comparative
ease. This w’ould not be the case either where the polar bodies
were not formed in line, or where, as in mammals, crossing-
over occurs in both sexes. Where crossing-over is absent in
certain regions, other types of inversions could easily become
established. On the other hand, translocations would be much
more readily established in any animals whose chromosomes
behaved like that of Oenothera and Datura.

The amount of inert material near the centromeres will also
have its influence, an increase favouring the detachment or
attachment of whole arms and viVc msri.

There is also evidence (Slack, unpublished, cited in Muller,
1940) that polyploidy may occur in some animal groups, e.g.
Hcmiptcra-Hctcroptcra. This will wholly alter the evolutionary
possibilities of a group.

In conclusion, Muller draws attention to the fact that in closely-
related wcll-analyscd pairs of forms different characters may
show different degrees of divergence. Thus serological differ-
ences, though usually agreeing with morphological ones, occa-
sionally give quite aberrant results — c.g. in regard to the relation-
ships ol D. hydei. Again, Drosophila simuJans and meLwogastcr
arc the least alike of five pairs of closely related forms in regard
to morphological characters, but most alike in regard to sec-
tional rearrangements. In the same study it was found that

SPECIATION, ECOLOGICAL AND GENETJC 371

Drosophila virilis and amerkana were least alike as regards meta-
phase chromosome-picture, but most alike in respect to their
fertility on crossing. It seems clear that the element of accident
is considerable. On the other hand, while complete parallelism
in regard to divergence in different characters is not to be found,
evaluation of the average divergence of numerous characters of
several different kinds does give a reasonable measure of the
relationship between related types.

Summing up, we may say tharspeciation in Drosophila appears
to havji been brought about mainly by the accumulation of
gene-mutations as a result of some sort of isolation. The isolation
operative appears to have been mainly geographical. Once in
existence, it will favour the origin of sterility barriers, which
in their turn will both permit and favour the increase of morpho-
logical divergence.

Certain types of sectional chromosomal rearrangements arc
also favoured by the genetic mechanism of the genus. Although
these have played some part in speciation, it appears to have
been essentially a secondary one, the consequence rather than
the cause of primary divergence.

On the other hand, the extent to which such rearrangements
have proceeded (often rendering it impossible to trace any
resemblance between the salivary chromosome-structure of
morphologically not very remote species) shows what a large
number of such differences accumulate within even a somewhat
uniform genus like Drosophila-, while the known fact of their
rarity compared with gene-mutation proves that the single-gene
differences between species must be enormously numerous. Gone
is any notion of species in higher animals arising by a single
mutation, or even by a few steps. Even closely-related species
will differ in scores, possibly hundreds of genes, and the longer
they remain in existence the greater are the number of genic
and sectional differences that are likely to arise between them.
Evolution consists m the accumulation and integration of very
numerous and mostly small genetic changes (p. 360).

In Drosophila as elsewhere, mode of life appears to modify
evolution. Drosophila pseudoobscura is more differentiated geo-

y }% evolution: THE modern synthesis

grapHcaUy (both as regards sterility-barriers and sectional re-
arrangements) than most species of the genus, which is almost
certainly to be ascribed to greater isolation between its local
groups (see pp. 6o, 6i). Li the production of its “races” A and B,
isolation appears to have been the first step, genically-determined
sterility the second, and sectional rearrangements the final step in
evolutionary divergence.

Even in Drosophila, where the species originally seemed excep-
tionally well delimited, careful analysis has revealed the existence
of all grades in spedadon, both as regards geographical sub-
spedation and the formation of sterility barriers.

All its species so far investigated carry large numbers of recessive
mutants in nature, and are thus provided with an adequate
reservoir of variability for future adaptive change and possible
further speciation.

It seems probable that spedadon in most large genera of h%her
animals is essentially similar to that in Drosophila, though with
minor difierences connected with consequential effects of their
chromosomal mechanism and mode of life.

The genus Crepis (hawkweeds) has diferentiated in an entirely
different maimer (see the monograph by Babcock and Stebbins,
1938; also Stebbins, 1940; Jenkins, 1939). Here various repro-
ductive peculiarities are at work which are available only in
higher plants, and we are given a very interesting picture of the
varying roles of selection, environment, and polyploidy in a
facultatively apomictic plant genus belonging to one of the most
advanced groups, the Compositae.

The old-world species of Crepis have basic haploid chromo-
sonie-numbers ranging from 3 to 7, together with polyploid
forms. Two of the American forms belong to this group, with
X = 7, one a circumpolar form found also in the old world,
C. nana, the other a closely-related type, C. elegans, which
appears to have diverged from C. tuma in America, and to have
become adapted to less extreme climatic conditions.

All the other American forms have the basic chromosome-
number X = II. There are seven distinct diploid forms (an = 22)

SPECIATION, ECOLOGICAL ANP GENETIC 373

together wilh a large number of polyploids, all apomictic (though
often with sHght facultative sexual reproduction), with diploid
chromosome-numbers ranging from 33 to 88, 44 being the
commonest. In addition, a few meuploids are found with
chromosome-numbers differing by i or at most 2 from a eupoly-
ploid number.

The evolution of this group of forms is deduced to have been
as follows. The ordinal ancestors were produced by hybridi-
zation between 4-chromosome and 7-chromosome old-world
types in the Siberian portion of the land-bridge which once
existed between Siberia and Alaska. This can be deduced from
the resemblance of the American forms to old-world species
with these chromosome-numbers. The hybrids underwent
chromosome-doubling to become fertile allopolyploids with
2n = 22 chromosomes. They did not spread westwards into the
old world, partly because they were there confronted with the
competition of the ordinal and already established types, while
the area to the eastward had not yet been occupied by Crepis;
and partly because the prevailing winds are westerly, and this,
in forms like Crepis with air-bome fruits, would encourage
easterly spread.

From a consideration of the morphological divergence of the
various American 22-chromosome species from old-world species
and from their present cHmatic and geological ranges, it can
further be deduced that different species were evolved at different
times— the two earHest during the Miocene, the next set (two
species) in the early Phocene, the last (four species) in late Pliocene
or early Pleistocene times.

These eight fertile species, though allopolyploids in ori^,
have acted fimcdonally as the diploid basis for later polyploidy-
in America. We can call them the American diploids. They
seem first to have become specialized to particular cHmatic con-
ditions, and the ranges of the earHer species were much restricted
by the cHmatic changes that followed.

As regards their later history, those forms fall into two groups.
The first consists of a single species, C. runcinata. This is the only
American Crepis adapted to moist stream-bank habitats. It thus

374 evolution: the modern synthesis

tends to follow valleys rather than mountain ranges. Accordingly
it has spread more widely to the east of the Rockies, where the
drainage basins are more continuous. Also, being ecologically
isolated from the other species with their preference for more
arid habitats, it has not hybridized with them to form allo-
polyploids. Instead, it has differentiated to form a polytypic
species or Rassenkreis, -with, well-marked geographical subspecies
in certain regions, and considerable variability in others. For
some as yet unexplained reason, it has not produced any auto-
polyploid varieties.

AU the other diploids appear to have hybridized to form
allopolyploids, sometimes with three or more components, and
in addition also to have produced autopolyploids. In some cases,
diploid geographical subspecies have also been differentiated. All
the polyploid forms are apomictic, some entirely, others prepon-
derantly so. The ancestry of the polyploids can in general be in-
ferred by the degree of their resemblance to the various diploids.

The formation of allopolyploid apomirts is favoured by
climatic and physiographic changes, which bring originally
separate species into contact. Once in existence, however, the
apomicts have less evolutionary plasticity, on account of their
total or considerable lack of recombination. Thus it is highly
probable that the intense environmental changes during the
glacial period will have encouraged the formation of many new
apomicts, while causing the extinction of the majority of those
produced in earlier periods.

The effect has been to produce what Babcock and Stebbins
call a largely agamic polyploid complex, in which all the original
and qualitatively differentiated diploid types arc connected by
an enormous array of intergrading forms. These differ from the
original diploids either in purely quantitative ways (e.g. effect
of polyploidy on size), or by combining their characteristics..
The divergent evolution of the group, which had given rise to
the ecologically specialized and morphologically distinct diplpids,
came practically to a standstill, to be replaced by gigas pheno-
mena, recombination of characters, and the segregation of
innumerable apomict “microspecies”. ’

; SP:ECIATIO,N,, ECOLOGICAL AND GENETIC,' ■ 375'

The; number of apomict types is very large near the- main'
centres of distribution, but much reduced in outlying areas* ■
New types are doubtless being constantly produced near the
distribution centres, and are still in process of being '‘tried out’^
so that many of them are likely to disappear. The outlying
forms are those which have survived and spread after earlier
origin, and therefore tend to have larger areas of distributioii.

The production of polyploids has undoubtedly enlarged the
range of the group as a whole, while the ranges of the original
diploids have in general been reduced by competition with
closely similar polyploids equipped with greater vigour.

Babcock and Stebbins also discuss the taxonomic treatment
of the group. They come to the conclusion that, while any
such agamic complex is in reality of a wholly different nature
from a group of non-interbreeding true species, yet for practical
reasons it is best to continue to employ the classical nomen-
clature. They accordingly recognize a series of “species’’, each
corresponding to each of the original diploid groups together
with its geographical subspecies and its autopolyploid derivatives,
and attach to each such species those apomicts which show a
preponderant resemblance to its diploid form. The Latin names
of the apomicts, however, are not regarded as forming part of
the nomenclature subject to the international rules; following
Turesson, they are preceded by the abbreviation apm,, £ot forma
apomictica. In addition, two other “species” are recognized, con-
sisting wholly of apomicts which are of such complex origin as
not to be attachable closely to any diploid type.

Tliis procedure is purely pragmatic and artificial, and, as
subsidiary terminology is evolved, may perhaps be superseded (see
also Turrill, 1938^, for Taraxacum).

One or two special points may be noted. In the American
agamic complex of Crepis, the pure autopolyploids are much
less widespread than the partially or wholly allopolyploid types,
contrary to the general rule in plants (see Miintzirig, 1936),
and their range as compared with that of the diploids is not
nearly as great as in such genera as- Tradescantia or Galium. This
is to be ascribed to an ecological reason— namely the preference

,37^ ' '■ . ^ E¥01.UTI0H":';^ THE MODEBN SYNTHESIS

of Crepis for arid habitats. The greater vigottr of most auto-
polyploids might thus cause the plants to demand more water
'than is normally available, while the frequent prolongation ol:
their flowering would, in regions of summer drought, also often
■be a disadvantage. Thus in Crepis she chief advantage of poly-
ploidy has come from allopolyploidy, wliich provides new
combinations of characters, permitting tlieir owners to invade
new habitats.

In general Babcock and Stebbins regard the production of
numerous polyploid forms as an evolutionary short cut by which
a genus may adapt itself more rapidly than by gene-mutation
and recombination to a rapidly changing environment. On the
other hand, in the long run, both polyploidy and apombds
constitute a barrier to the more important evolutionary process
of divergent specialization, the former because the duplication
of gene-pairs makes it more difficult for recessive characters to
come into action, the latter because sexual recombination is
impossible.

Furthermore, the immediate plasticity conferred by allopoly-
ploidy will only continue so long as the sexually reproducing
forms of a complex continue to be present and to cross. Thus
in western America, where Antemaria exists in a polyploid
complex still containing sexual as well as apomictic polyploid
forms, it is an aggressive and dominant form. In Newfoundland,
on the other hand, the genus is represented only by obligatory
apomicts. These are all relict forms, not at all aggressive, and
often very localized in their distribution (Fcmald, 1933). Even-
tually, groups of apomicts separated from their sexual ancestors
will be doomed to extinction as they can no longer meet changing
conditions.

One effect of polyploidy is to spread the polyploids at the
expense of the diploids. Thus, while the bringing together of
diploids by climatic change will encourage an outburst of allo-
polyploid forms, the very success of the polyploids will, if con-
ditions later become stabilized, gradually remove the conditions
in which the continuance of their new formation is possible.

Babcock and Stebbins consider, first, that the present liigh

SPEGIATION, ECOLOGICAL AND GENETIC 377

incidence of polyploidy in liiglier plants is a consequence of tibe
extremely large and rapid climatic changes of the Pleistocene
and Recent periods, which have not only promoted allopoly-
ploidy by bringing diploids together, but have enhanced the
evolutionary value of polyploidy as a short cut to meet rapidly
changing conditions. Secondly, that the prevalence of apomnds
in such groups as Gramineae, Rosaceae, and Compositae is not
due to any peculiarity of their germ-plasm, but to the fact that
they happen to be groups which in geologically recent times
were rapidly evolving in such a way as to produce numerous
young and vigorous agamic complexes.* And thirdly, that all
such agamic complexes are destined eventually to decay untd
they are extinct or are represented by a few relic types only,
while new agamic complexes may be formed later by those
groups which are at the right evolutionary stage when the next
rapid change of climatic conditions takes place.

In a later paper Stebbins {iQiob) discusses the taxonomy of
some forms related to Crepis in the tribe Cichorieae, notably the
somewhat primitive genera Soroseris, Dubyaea, and Prenanthes.

Both the first two appear to have 2n = i6 as their diploid
chromosome-number, though one probable tetraploid is known.
Apparently in primitive members of the tribe, quite large changes
in general structure and macroscopic characters are accompanied
by comparatively slight changes in chromosomal morphology
and structure, while the reverse is usually the case in the more
specialized forms. Dubyaea probably dates from the Cretaceous
and later became restricted to a “refuge” in the Sino-Hunalayan
area, having been exterminated elsewhere in competition w' lth
its more aggressive descendants. This confirms Matthew’s view
(1915) that primitive types tend to be preserved near the margins
of the range of a group.

One section of the genus appear to have given rise to Prenanthes
(probably as far back as the early Tertiary), Lactuca, Hleraciunt
and Crepis itself. Soroseris must also have been derived from

* The fact that Crepis runcinata has, owing to its ecological peculiarities,
escaped from the agamic complex and imdergone a more normal type of evolu-
tion, is another proof of thi«

378 EVOiUTlON: THE MODERN SYNTHESIS

Dukyaea, but probably from a section now extinct, and perhaps
polypbylctically. It also is restricted to the Sino-Himalayan area.
It apfKJars to have originated in Tibet, at a time of desiccation,
presumably in the later Middle Tertiary. During the Pleistocene,
glaciation isolated various groups, thus providing the basis for
the differentiation of the numerous closely related species and
subspecies now found in the genus. It is interesting to find how
different die mode of evolution has been in these primitive
genera from that in Crepis.

The state of affairs in Crepis may be briefly contrasted with
that in Tulipa, recently monographed by A. D. Hall (1940).
In this genus there appears to have been considerable diver-
gence, not associated with polyploidy, into a number of main
sections. Within the sections, however, autopolyploidy has been
frequent, giving 3n, 4n and occasional sn forms, the anisoploids
showing vegetative reproduction. In some types, tctraploids have
originated separately in different parts of the range, giving forms
which show sUght quantitative differences as weE as size-differ-
ences associated with the chromosome-doubling, In some types
there is considerable geographical differentiation, giving rise to
forms which zoologists would certainly recognize as subspecies.
There is no evidence of aEopolyploidy or reticulate evolution.
In the garden tulips polyploidy is unknown, apparendy owing
to their large chromosomes (Darlington, 1937, p. 84).

SimEarly, the state of affairs in Drosophila may be profitably
contrasted widi that in the bird genus Zonotrichia, one of the New
World finches or “sparrows (see Chapman, 19406). It comprises
only five species. Four are North American, one confined (in the
breeding season) to a central region of northern Canada, another
to western and southern Alaska and neighbouring islands. A
tliird {Z. alhicoltis) is essentiaEy an eastern species, breeding as
far south as Wisconsin and Pennsylvania, but reaching almost
to. the Arctic and the Pacific oceans in the N.W. None of these
three species, not even the last-named with its large range, shows
any subspeciation. The fourth, however, with a larger (and
rather more westerly) range, from Greenland and the St.
Lawrence to the Pacific, and from the northern tree-limit

SPECIATION, ECOLOGICAt AND GENETIC

379

to southern California, has differentiated into 4 well-marked
subspecies.

Finally Z. capensis, a Central and South American form, boasts
no fewer than 22 subspecies. Chapman considers that it was the
southernmost representative of this originally northern genus,
was forced southwards across Central America by the onset of
glacial conditions, and then continued to spread wherever the
climate was cool enough, until finally it colonized all suitable
habitats in South America, down to Cape Horn. Its distribution
now covers 4,000 miles from N. to S., 3,000 miles from E. to
W., and 15,000 feet of altitude — a much larger range than that
of any other member of the genus.

Once it reached South America, its further spread must have
been due not to chmatic influences, but solely to natural increase,
which appears to have been rapid in the new territory thus
made available to a hardy form differentiated in the more rigorous
conditions of the northern hemisphere.

The original migration through Central America must have
been at sea level, but with the post-glacial amehoration of climate
it moved up to higher altitudes, thus becoming restricted to
discontinuous upland areas in various more tropical parts of its
range. There are two exceptions: certain groups early colonized
some islands on the Pacific coast of Central America and othen
off the north coast of South America, and thus could not move
to higher altitudes when the climate grew warmer. The forms
on the South American islands overlap with those of the adjacent
mainland in character, but are paler, and distinct enough to
merit subspecific naming; but Ae Central American insular
populations show no visible distinctions from the neighbouring
mainland forms, though separated from them by a minimum
of 2,500 feet of altitude. It would, however, be of great interest
to see whether they show special physiological adaptations to
the unusual climate of their enforced habitat.

Some further points of interest are as follows. AH forms of
the species appear to be residents, except for the southernmost
subspecies, which is definitely migratory. Here is a good example
of local adaptation, which must be of recent origin, since this

38 o ,'e.voxution,: "■..the- mooerm. ^synthesis ,

siibspccics iiiust clearly have bceii the last to diferciitiatc. It also
possesses the longest and the most pointed wing of any of the
subspecies. Though this must in part be regarded as adaptive^
Chapman points out that it is in part the culmiiiation of a cliiic
in wing-size, which increases more or less steadily southwards
through the continent, and is presumably a “correlated character'',
non-adaptivc per sc. Adaptive change would here have been
superposed on noii-adaptive in the migratory subspecies.

There is also a general N.-S. iiitcrgroup sizc-cline within South
America, but there arc exceptions to it, and there is considerable
independence in the variation of the size of different parts. The
different subspecies show a good deal of geographical variation
in song.

An interesting barrier is found at one spot in the mountahious
interior of Venezuela, Here isolation has allowed a subspecies
to differentiate from the main Venezuelan form; but the table-
land at the summit of Mt. Roraima is separated from the area
below by a 1,400-ft. vertical cliff, and this in turn has permitted
the summit population to differentiate into a darker form just
distinct enough for subspecific recognition.

Accidental “drift” in isolated populations has also clearly
contributed to differentiation. One curious feature is that, whereas
all the North American species have some yellow on the bend of
the wing, this is present in only four of the subspecies of capensis
— one form from the Antilles, and three adjacent subspecies from
the centre of the cast coast of the continent. In the central of
these latter the yellow is all but universal, but in the subspecies
to N. and S. it is sporadic, and in one of them only faint. In the
distant Antilles race it is universal but faint Here, as Chapman
points out, we appear to have the partial resuscitation (or less
probably the preservation) of an original generic character which
has been lost in the main body of the species.

In tliis genus, differentiation thus seems to have been brought
about via geograpliical isolation followed by adaptive and
accidental character-divergence. We have the somewhat puzzling
fact of the absence of subspeciation in one wide-ranging Nortli
American form, a moderate degree in another, and a high degree

SPECIATION, ECOLOGICAL AND GENETIC

381

in the one South American spedes. This last fact is probably due
to the very large range of habitats thrown open to the spedes
once it had been pushed through the Central American bottleneck
by the onset of a glacial climate. Zomtrichia shorn no obvious
trace of the genetic isolation to be seen in Drosophila; and though
genetic analysis (if it were possible) might possibly reveal that it
had occurred, it cannot well have played more than a very
minor role in this genus.

As a parallel illustration from plants, of the principle that
differentiation may vary considerably with local conditions (see
also the case of Crepw), 'we may take the peonies, Paemia (Barber,
1941). The pre-glaciai'spedes appear to have been diploids. In
Europe and the Caucasus, the majority of modem species are
tetraploids, but ip China and Japan there are only a few tetra-
ploids. The reason appean to be as follows. In the former area,
die diploids were for geographical reasons unable to retreat far
to the south before the advance of the ice, and they were exter-
minated except in a few “refuges”. Any tetraploids which arose
then had a field almost free of competition, in addition to any
advantages due to extra hardiness (p. 337 )- In the ^ast,
however, the original diploids simply retreated southwards before
the ice, and advanced again in mass on its retreat,* so that there
was much greater competitor-pressure against any tetraploid
forms (cf. the case of Crepis, 373)- Finally in California, for
reasons unknown, structural hybrids of the Oenothera type, based
on segmental interchange and balanced lethals (pp. po- 3 ^ 9 ) are

Postscript— Since first printing E. Mayr has published his v^uablc
Systematks mid the Origin o f Species (New York 1942). Reference
must be made to his important conclusion that, m higher animals
at least, widi the exception of “biological” differentiation (my p.
295), the only factor permitting group divergence is geographical
isolation; neither ecological nor genetic isolation is ever primary.
1 am bound to say that Mayr has convmced me on this point.

* A ^milar mass retreat and advance was possible for the pre-glackl forests
in North America, but not in Europe, leading to a great impoverishment of
the European forest tree flora as compared with that of the U.b.A.

CHAPTER 7

SPECIATION, EVOLUTION, AND TAXONOMY

I- Different types of speciation and their results . . p. 3^2

2. Spedcs-fomiation and cvoludon 3^7

3. Modes of speciation and systematic method . . . p* 390

I. DIFFERENT TYPES OF SPECIATION AND THEIR RESUiTS

So far, we have been considering the different methods by
which species may originate. It should be remembered that the
type of origin may have effects upon the subsequent type of
variation shown by the species. Thus in vegetatively reproducing
polyploids, variation will be much restricted since no recom-
bination of mutations can occur. In parthcnogcnctically repro-
ducing allopolyploids, on the other hand, crossing-over may
give rise to purc-breeding segregants (p. 334), so that we may
expect a number of sharply defined but closely related purc-
breeding types. In balanced-lethal hctcrozygote species, crossing-
over will also operate to give large apparent mutations. Sexually-
reproducing polyploids will show a diflferent type of variation
from diploids, since each gene will be represented in four or
more identical or closely similar forms instead of two. This
will give a greater supply of similar mutations and thus a greater
evolutionary flexibility, but less opportunity for single mutations
to exert any considerable eficct. Darlington (1933), looking at
the matter from the comparative, not the evolutionary, point
of view, distinguishes six kinds of species according to their
genetic-reproductive mechanism: (i) the habitually self-fertilized
diploid; (2) the habitually cross-fertilized diploid; (3) the sexually-
reproducing fertile polyploid; (4) the -mixed species containing
both diploid and polyploid forms; (5) the complex-hcterozygotc
species (balanced lethal type), as m Oenothera', (6) the clonal
species not reproducing sexually at all. This last category could
be divided furdier into the parthenogenetic forms showing

SPECIATION, EVOLUTION, AND TAXONOMY 383

asexual segregation, and the rest which do not. To this list we
may add (7) the subsexual species like Rosa canina (p. 351);
and (8) those animals such as Hymenoptera (and certain beetles:
A. C. Scott, 1936) with diploid females but haploid males. Still
further types might be added, e.g. those with close linkage
promoting polymorphism (p. 99). Darlington concludes his
paper: “Genetics leaves no doubt that each of these types will
have certain characteristic properties of variation. It is for the
taxonomist, armed with the cytological information, to find
out what these are.”

Apart from this, selection may be expected to act in quite
different ways and with quite different intensities according to
the method of speciation. Our analysis has enabled us to dis-
tinguish in principle between the causes of their isolation and
those of their divergence — ^between the factors making for
isolation between groups within an original single species, and
those making for diference in the structural and functional
characters separating new species from their parents or nearest
relatives.* Groups separated by geographical isolation are origin-
ally species only in posse. Thek separation into good species is
a subsequent process, accompanying the process of character-
divergence. This divergence is normally slow, but occasionally,
as on oceanic islands and other places where the intensity of
selection is relaxed, it may be much more rapid and more
extensive than usual.

Elsewhere, as apparently in the case of Drosophila simulans
and D. melanogaster, the isolation is of such a nature that the
two groups must be regarded as separate species even when
stiH almost indistinguishable in any characters save those which
isolate them. Indeed it is conceivable that in such species,, character-
divergence may not subsequently occur: in Drosophila simulans
it has at least been minimal. At the opposite extreme are those
cases in which the factor inducing isolation simultaneously pro-
duces character-difference, of an order which will — or at least
may — ^be accepted as of specific magnitude by the systematise

* Plate’s (1913) chapter on isolation is still very well worth reading in this
connection.

384 evolution: THE MODERN SYNTHESIS

This is so in Spartina townsendii, and most cases of convergent
and reticulate species-formation. Further character-divergence
may, of course, occur later, as with Galeopsis tetnihit (p. 341),
but this is irrelevant to our argument.

From the standpoint of the mode of action of natural selection,
species will then fall into two contrasted categories. On the one
hand, we have those in which natural selection can have had
nothing to do with the evolution of the basic specific characters,
but merely acts upon the species as given, in competition with its
relatives. These include all species in which cliaracter-divergence
is abrupt and initial. On fixe other hand, we have those forms
in which character-modification is gradual. Here natural selection
may, and on both deductive and inductive grounds often does,
play a part in producing the characters of the species (and by
characters we, of course, mean not only those which are employed
by the systematist, but all those which do in point of fact dis-
tinguish it from its nearest relatives). These include not only all
forms in which the separation of groups occurs by geographical,
physiological, or ecological isolation, but also those in which
the initid separation is genetic but involves no visible differen-
tiation.

From the point of view of the intensity of selection, the
successional evolution of species will, ex hypothesi, be directed
by selection wherever the trend of evolution is' towards some
adaptive specialization (p. 494). Then it is clear that groups
separated ecologically will be exposed to a considerable intensity^
of selection to adapt them fully to their different modes of life
When they overlap spatially with closely-related groups, selec-
tion may also be expected to act upon them to produce barriers
to mating (p. 387}. This latter mode of selection will not operate
in the case of geographically separated groups, but selection
towards divergent general adaptation will occur if the environ-
mental conditions in the two areas are different. When, however,
the two areas are similar in the environment they provide, there
win be reduced scope for selection, and if divergence occurs, it
win be primarily of an accidental and often of a biologicaUy
non-significant nature. This will also apply to species which

SPECIATIOH, EVOLUTION, ANO TAXONOMY ■ ■ 385

overlap spatially, but owe their origin to a gciietical mode of
separation which does not cause visible differentiation, such
as large inversions or asexual segregations: in the former
case, however, selection should operate, as with overlapping
ecologically divergent species, to produce barriers to inter-'
breeding.

We may present the chief results of the two previous chapters
ill tabular form (see p. 386).

In the first column wc distinguish between the four myor
types of species-formation — succcssionai transformation, diver-
gence, convergence as a consequence of spccics-crossing, and
reticulate evolution.

In the second column we distinguish the main factors leading
to the separation of two species. In succcssionai transformation,
time is the factor at work, hi geographical, ecological, and
physiological divergence there is always sonic topographical
isolation. Wc may call this type of separation spatial, contrasting
it both with the temporal and the genetic; but the scale of the
spatial factor is different in the three sub-types. If v/c preferred,
we could equally well call it eiiviromnentai, since it is concerned
wntli something outside the organism, in contrast with constitu-
tional separation, depending on genetic factors.

Genetic separation operates in the remainder of the divergent
and in all die convergent types.

In the third column we note whether the actual formation
of species, regarded as distinctive or intersterilc groups, is gradual
or abrupt; and in the fourth we consider the same distinction
with regard to their visible differentiation. It should be noted
that the two do not always run parallel. In column 4, the phrase
initidly abrupt means that some visible difference occurs with
the first abrupt origin of the species, but that further gradual
divergence may supervene later.

Finally, in the last column we consider the actual barriers to
fertility, including under these barriers to cross-mating between
the pairs of species. Consequential implies that these barriers are,
m some way (p. 359; Muller, 1940), the consequence oi the differ-
ences that have gradually arisen between the two species. Initial

evolution; the mouesn synthesis

lat the new species is automatically, by its genetic
m, unable to cross with its nearest relatives, or that

the offspring of such a cross are either infertile or of reduced
fertility. Selective implies that selection will operate to erect
special barrien to cross-mating or cross-fertility.

sfeciation; evoeutioh, anb taxonomy

. 2, SPECIES-FORMAHON AND EVOLUTION'

Since the origin of species has occupied the centre of the biolo-
gical stage since the time of Linnaeus, it is to this problem that
we have devoted the bulk of the two previous chapters. One point ,
at least emerges clearly; if Darwin were writing to-day he would
call his great book The Origins^ not The Origin, of Species,

But we may conclude by looking at the matter from a still
broader point of view, in the perspective of evolution in general.
Evolution may be regarded as the process by which the utiliza-
tion of the earth’s resources by living matter is rendered pro-
gressively more efficient. Eatly in the process, living matter
became organized into cells, evolved a particulate hereditary
constitution arranged in chromosomes, and developed the sexual
process. The reason why the sexual process (which in its inception
was not connected in any way with reproduction) occurs in the
great majority of animal and plant types aUke, is that it con-
fers a greater potential variability on its possessors, and there-
fore a greater plasticity in evolution. It does this by being
able to combine mutations which have occurred in different
strains, and which in an asexual form would have to remain
separate.

The exploitation of the earth’s natural resources progressed
in two complementary ways — ^by improvements in basic mechan-
isms of exploitation, and by adapting a given basic mechanism
to every possible kind of environment. We shall discuss the
former more in detail in our chapter on Evolutionary Progress:
here we may give as illustrative examples the colonization of
the land by plants, and the evolution of considerable size and of
rapid locomotion by means of limbs in animals.

We then come to the second method. The green plant exploits
light and air and water in every conceivable habitat, appearing
here as floating diatoms in the surface layer of the sea, there as
giant forest trees, here as prairie grasses, there as duckweed in a
pond. Again, in animals the fish type exists in the deep sea, in
its surface layers, on sandy and rocky shores, in rivers, in lakes,
in caves. There is operative a selection-pressure forcing life to

388 eyolutioh: the modern synthesis

occupy every geographical area and every ecological niche
within each area.. (See also Chapter 8).

Now it is clear that, living matter being what it is> mere
diflference will quite soon make breeding impossible between
diverging groups. Chromosomes will n ot pair at meiosis unless
reasonably similar, and unless they pair at meiosis, sexual repro*
duction cannot occur; and see p. 359. With still further divergence,
the two sets of chromosomes are unable to combine in the
work of building up a new organism: hardly any case is known
of offspring resulting from a cross wider than intergeneric*
Living matter thus inevitably becomes broken up into a large
numkr of non-interbreeding groups, the majority of which
coincide with taxonomic species.

On the other hand, there would seem to be no a priori reason
why a single species should not range over a very wide geo-
graphical area, varying somewhat from region to region, but
with aU such varieties forming, actually or potentially, part of
one interfertiie group, nor any a priori reason why more than
one species of the same family or genus should occur in the
same ecological habitat

However, we find that in neither case is our expectation
justified: very large numbers of species occur for whose existence
there seems at first sight no reason or meaning. On looking
further into the matter, we see that this depends on two sets of
facts, one connected with the relation of the organisms with
their environment, the other with their genetic basis. The
environment is subjected to changes which create barriers
between one region and another, and thus isolate groups belong-
ing originally to the same species. And complete isolation permits
differences, both of an adaptive and of a chance non-nrilitarian
character, to accumulate relatively fast in the two groups, until
in many cases they become new species.

Then the chromosomal basis of heredity is subject to accidents,
such as inversion, segmental interchange, hybridity, and poly-

* Dr, W. B. Turrill in a letter states that the widest cross he knows is between
die rashes Cyperus dmtatus and Rhynchospora capitellata^ which are placed in
different sub-families of the Cyperaceae. The hybrid is entirely sterile.

SPECIATION, EVOLUTION, AND TAXONOMY 389

ploidy, which sooner or later will reduce or abolish fertile
mating between the new and the old type. 'in this way large
numbers of new species essentially similar to those from which
they arose are brought into being, and the new and the old
come to compete with each other in identical or (often as the
result of subsequent miration) in overlapping habitats.

The formation of many geograpliicahy isolated and most
genetically isolated species is thus without any bearing upon
the main processes of evolution. These latter, as we shall see in
later chapters, consist in the development of new types endowed
with mechanisms of higher all-round biological efficiency; in

B¥OLtJTIONl ' THE . MOBMIN. . SYNTHESIS;

3* MOBIS OF SFECIATION ANB SYSTEMATIC MBTHOB

HaYing now discussed modem work dealing with the difFerent
modes of speciation, we must now consider its bearings upon
taxonomy and systematic ■ method. Historically, we may -dis-
tmgiiish three main phases in the history of modem taxonomy,

each with a different principle serving as its main philosophic
basis (see TurriU, 1936; Gilmonr, 1937)- In the first or Linnaean
'period, the underlying principle was the separate cre^mon of
species. In the second or Darwinian phase, it was the dortrine
of descent with modification. And in the third, the Mendelian
period upon which we are now entering, it is selection based on
the cytogenetic theory of particulate inheritance and mutation.

Let us amplify these points a Httle further. Linnaeus, in the
latter part of his career, was a firm upholder of the immutability
of species: “Species tot sunt, quot formae ab initio creatae sunt.”
This doctrine of the fixity of species was in one aspect the
rationalization, or at least the reflection, of the practical need
for identifying plants for medicinal purposes (see p. 263). Once
accepted, it lent itself to the furtherance of easy identification.

If species are immutable and distinct entities, the chief aim
of systematics becomes that of distinguishing between Aem.
This naturally led to the codification of artificial “laws” and
“systems”, of which that of Linnaeus for higher plants is the
classical example. This wzas really no more tlwn a key to the
identification of larger entities, based on arbitrary and for the
most part biologically almost non-significant features such as the
number of stamens and pistils.*

The artificiality of such unnatural systems was in part corrected
by an instinctive logic which led man to search for a basis of
classification that should take into account both the number of
the points of resemblance between groups, and the intrinsic
importance of the points of resemblance chosen as diagnostic.
We can accordingly trace the abandonment of purely artificial
systems for those based on general likeness. Still later, as it was

* On lower taxonomic levels, such as the generic and specific, Linnaeus’s
common-«ense and natural intuition led him to remarkably modem groupings.

SPECIATION, EVOLUTION, AND TAXONOMY 39I

realized that superficial resemblance (as between a porpoise and
a true fish) may mask basic difference, we may see the substitution
of likeness in fundamental structural plan as chief criterion, in
place of mere superficial likeness. Pre-Darwinian nineteenth-
century classification, as practised by Goethe, Cuvier, Oken,
Owen, T. H. Huxley, etc., worked on this assumption.

But although this method, at least for larger groups^ was
identical with that practised in the latter half of the century,
it lacked any real theoretical basis grounded in biological justifi-
cation. The analytic but less speculatively-minded, like Huxley
(e.g. 1853, 1854), simply assumed that structural homology (or
common archetypal plan) was the right key to unlock classifi-
catory secrets; the idea that it was right because it implied
genetic relationship did not enter their minds, or at least was
not allowed to enter their conscious minds, until after the
pubheation of Darwin’s Origin in 1859. The more theoretically-
inclined, such as Goethe and Oken, regarded the existence of
structural plans common to a large number of animals as evidence
of some form of planning in the act of creation. In extreme
form, this theoretical view found the basis of homology in the
existence of a Hmited number of archetypal ideas in the mind
of the Creator.

With the coming of the Darwinian epoch, however, all this
was changed. Homology, instead of being essentially a descriptive
term implying nothing more than the sharing of a common
archetypal plan, became an explanatory term implying the
sharing of a common plan on account of descent from a common
ancestor. The basis of classification became, in theory at least,
phylogenetic. Degree of resemblance was taken as index of
closeness of relationship, and taxonomic categories were defined
on the assumption that each represented a branch of higher or
lower order on a phylogenetic tree.

Tim way of looking at the facts provided what was on the
whole a very satisfactory basis for the delimination and arrange-
ment of larger classificatory groupsjdown to orders, sub-orders,
and even families: but it was not always easy to apply it to the
minor systematics of genera and species.

392 evolution: the modern synthesis

In practice, minor systematics was stffl ruled by an oudook
which in some respects remainea Lirmaean In spite of the
theoretical beUef that species were mutable, they were usually
defined by the aid of criteria which tacitly assumed immutability,
or by arbitrary characters frankly based on mere convemence.
This^point of view is stUl employed by many taxonomists
to-day, and the result is often an arbitrary compron^e between
practical convenience and the desire to give a spafic name to
every recognizably distinct form. This is perhaps less so m
zoology, where subspecific naming in accordmce with the
principle of geographical replacement is now the pracnce m
most well-worked groups. Even here, however, as mentioned
in the section on clines (p. 206), subspecific nam^ are often
allotted on the basis of an arbitrary degree of dtference m a
continuous series, not on that of the existence of natural self-
perpetuating groups with relatively uniform c^acters.

In botany, however, procedure is often still quite arbttrary.
To take one recent example, Cowan (i 94 o) divides the rhodo-
dendrons of the sanguineum sctks into eight species and thirty-
eight subspecies. This is done on certain arbitrary diagnostic
characters. “It must now be decided whether e^h of these eight
groups is to be regarded as a single variable species or as a section
including a numter of specific units.” ... “It must be under-
stood t-hat the species vary within the widest limits in characters
not takpTi as diagnostic. The same argument applies with even
greater force to the subspecies.” No attempt is made to employ
geographical distribution as a taxonomic character. Alfho^h
there is ''abundant evidence of the distribution of these (diag-
nostic) character upon mendelian lines”, and “many of the
possible combinations do occur in nature”, there is no discussion
as to whether this state of affairs may not be due to hybridization
and reticulate evolution; the only criteria used are morphological
separability and practical convenience: Even if all these variants
can rightly be regarded as species, the multiplication of spedfic
names to tliis extent is so obviously undesirable that one turns
at once to the alternative course of modifying the standard. It
is equally undesirable to regard all the plants within this group

SPECiATION; EVOLUTION, AMD "■ TAXONOMY , • ■393''

as forms of a single very variable species, a not inireasoriable
view, but they diifer too widely/' In other words, taxonomy in
cases like these makes no pretence of describing the facts of nature
concemmg the distribution and relationsliips of natural groups,
but is concerned solely with the arbitrary distinction of forms.

It is clear that distinguishable forms should receive some
designation; but this should not be a specific or subspecific Latin
name unless there is some ground for supposing that the dis-
tinguishable form is also a natural group-unit Other forms
should be distinguished by some type of subsidiary nomenclature,
as Turrill (1938a) proposes.

Botany also lags behind zoology in another point of taxonomic
practice, which, though small, makes for convenience. I refer
to the convention by which all specific names arc spelt with a
small initial letter. This is now universal in zoology, and I have
deliberately adopted it in this volume. The elaborate conventions
of botanical practice occasionally make for confusion and liave
nothing to recommend tliem save historical tradition.

The value of employing every possible type of character in
taxonomy is illustrated by recent work on the related plant
genera Hebe and Veronica. The two genera were separated
according to the mode of dehiscence of their capsules, and on
this basis a number of New Zealand species were assigned to
Veronica. However, they have now been found (Frankcl, 1941)
to have the same basic chromosome-numbers as Hebe {two
polyploid scries, with x = 20 and x = 2i), in this diifering
from all typical Veronica species. Re-examination of the capsule
then showed that the mode of dehiscence is much more similar
to that of typical Hebe. The species have accordingly been trans-
ferred to Hebe. Similar corrections of faulty taxonomic observa-
tion by new methods, in this case the utilization of chemical
data on pigments, have been made by Ford (1941) i^ Lepidoptera.
Metcalf (1929) has pointed out the value ofparasites for taxonomic
purposes.

In spite of all efforts to draw the taxonomic consequences
of the geographical replacement of forms, efforts dating from
Gloger's pioneer work in the second quarter of the nineteenth

394

E¥0X1JTI0N t '■ THE . MODERN , SYNTHESIS /;

century and continued by such men as Allen and Gi^ck m
the ’70’s, Eimer in the ’8o’s, and Kleinschmidt a«d the Saxasms
in the ’90’s, the detennination of species down to the beginning
of the present century was usually undertaken on the assumption
that they were aU well differentiated by a series of diagnostic
characters, and separated from their nearest relatives by sharp gaps.

Determination was made almost exclusively, and often rather
arbitrarily, on the basis of morphological characters of structure
and appearance. As research brought to light more md more
geographical or other forms, populations which could be clearly
distinguished from the populations of other areas were generaUy

accorded specific rank. _ .

The last decades of the period of phylogenetic classification,
roughly from the beginning of the present century onwards,
may be distinguished as a definite sub-period, chara.cterized by
the use of geographical distribution as a taxonomic criterion,
in addition to morphological characters. From what we have
just said, it should be clear that this also meant the abandonment
of the last traces of a subconscious ‘Xinnaeism , and the adoption
of a thoroughgoing phylogenetic outlook, in mhior as well as
in major systematics. In the battle between the spHtters and
the ‘lumpers'^ the ‘‘sphtters’’ represented the last survival of
the Linnaean outlook, the ‘lumpers'' the geographical phase of
the Darwinian.

The first result of the refinement of detailed systematic methods
was thus to force the geographical criterion into prominence
and to introduce the Darwinian idea of plasticity into the
taxonomies of species.

To-day, however, the discoveries of cytology and genetics,
together with the mass of detailed systematic data vrhich they
are illuminating from a new angle, have shown us that we must
adopt addition^ and in a sense other criteria.

A classification based on the idea of phylogenetic descent
must at best remain highly speculative, for, save in a few fossil
lineages, we do not and cannot know the actual course of events
in the evolution of a group. In most groups, the only data we
possess on which to base our classificatory scheme, are those

iPtCIATION, EVOLUTION, AND TAXONOMY 395

concerning the species, subspecies, and genotypic variants as
they exist at the present time, for these are the only groups with
concrete biological existence. These obviously represent the
results of evolution, but often tell us Httle about its past course.
From what we now know with regard to the different methods
by which new species are produced, and the genetical and
cytological mechanisms underlying their production and main-
tenance, we can see the problem in a new light. We are beginning
to realize that a new basis for classification will be necessary for
dealing with minor systematic diversity, although the phylo-
genetic method will remain applicable to major groups.

Let us see in what main ways a scheme with such a genetic
basis for taxonomy wiU differ from one with a phylogenetic
basis. In the first place, we have the undoubted existence of
parallel mutations (see p. 510). When these occur and are pre-
served in stocks which are already specifically distinct, the
Darwinian concept of homology breaks down. For the homo-
logy, though perfectly real, no longer implies descent from a
common ancestor showing the common feature. Two white-
eyed mutant strains in two species o£ Drosophila are not descended
from any common white-eyed ancestral strain; and the same
doubtless holds for various wild-type characters of related species.
It is true that where a number of separate characters are involved,
as in the plan of construction of the body as a whole or of any
complex organ, the phylogenetic concept of homology will still
hold. It is impossible to maintain the independent evolution, on
more dian one separate occasion, of such structures as the penta-
dactyle Hmb of land vertebrates, or the crustacean appendage,
or the chordate notochord. Phylogenetic classification based on
the idea that the possession of such organs by a number of
organisms implies their descent by modification from a common
ancestor remahis as vaHd to-day as it did when the principle
was appHed by Kovalevsky to prove the vertebrate affinities of
the Tunicates. In plants, on the other hand, the organization of
the body is on the whole so much simpler that structural plans
of such complexity as to rule out close parallel evolution are
rare; it is for this reason that the phylogeny of plants is much

,396' ■ ' EYOIUTIOH-:- THE' M.ODIEN SYNTHESIS

inofe tmcertaiii than that of higher , aniioalsj and botaMsts as a

wkole correspondingly more pessimistic than zoologists as to
the possibility of phylogenetic classification in general.

hi certain long-range evolutionary trends in aninialsj parallel
changes appear to Have played a greater part than was earlier
supposed. It is for instance probable on a priori grounds, and
f-prrain on the basis of fo^il evidence, that many adaptive features
in a type undergoing specialization are due to the selection of
parallel but independent mutations. This is brought out clearly in
the case of the horses (Matthew, 1926). Here, quite distinct lines,
somc wHch cventually become extinct, show the same
general changes, though some may be in advance of the average
in one specialization (e.g. teeth), and behind it in another (e.g.
feet). Something similar occurs in the more finely-documented
evolution o£ Micrasfer (p. 32)- Presumably the general direction
in which selection-pressure is being exerted on the group
remains constant, and thus all mutations and reconihinations
favouring change in this direction are selected. It is not necessary
(and indeed highly improbable) that the parallel mutations
should be strictly homologous, in the sense of being changes
in the samp gene; the parallelism of evolution and consequent
upset of the classical concept of homology will occur just the
if they merely exert similar effects.

It is posable that parallel specializations or parallel progress
of this sort occurs also in larger groups. W. E. Le Gros Clark,
for instance (1934), believes that it has played a large role in the
evolution of the Primates as a whole.

It is, however, in minor systematia that the greatest difficulties
occur. In die first place, we have the fact that parallel mutations,
jriHiiding a number that are fuHy (genically) homologous, occur
in related species of Drosophila and other organisms. They are
conspicuous where fixed in domesticated forms (see Haldane,
19270, on mammals), but occur also in wild populations. This
makes natural the presumption that certain characters actually
found established in some of the species owe their or%in to
parallel mutation and not to common descent. It is clear that
the distribution, among a group of related species, of charaaers

SPECIATION, EVOLUTION, AND TAXONOMY 397

due to parallel mutation m%lit be quite different from a distri-
bution dependent on phylogeny. Similarity of mode of life,
with consequent preservation of similar mutations, would be
more influential than common ancestry (though parallel mutation
is only likely to occur in closely related forms). Sturtevant (1939),
for the Drosopliilinae, is probably the only taxonomisc who has
consciously endeavoured to discount this possibility (p. 357).

Quite frequently characters wiU form a mosaic pattern.
Character A will in one species be combined with B and C,
in another with B and D, in yet another with C and E, and
so on (e.g. in Drosophila; p. 370). In such cases we must be content
to let the phylogeny of species elude us.

In general, taxonomic “relationship” will in many cases be
quite different from relationship in human affairs, as between
members of a large family. In the first place, the one is essen-
tially an affair of groups, the other of individuals. In the second
place, the facts concerning mutation, such as its recurrent nature,
and indeed the necessity (if we are to account for the variance
actually found in nature) for some recurrence to balance die
wastage due to random loss of mount genes from the germ-
plasm, make it clear that while human relationship is based on
physical continuity by reproduction, uxonomy is essentially con-
cerned with the number of characters or genes shared in common.

Let us amplify these points a htde. The taxonomist is not
concerned, or is concerned only in a very minor degree, with
rare individual variants. These may, in certahi cases, constitute
the raw material out of which taxonomic units are shaped, as
with dominant melanism in moths (p. 93), but in diemselves
deserve notice, if at all, merely as “aberrations” from the type
of the group. It is only when a group is involved, whcdier in
the form of a single localized unit, multiple localized units, or
a distinct and common type scattered through the population
(as with genetic polymorphism: p. 96) that taxonomy is involved.
In human relationships, on the other hand, we deal primarily
with individuals; A.B. is the son of CT)., the nephew of M.N.,
the-cousin of X.Y.

And the basis of these human relationships is reproductive

39'8.' '■ ‘■.EVOLUTION..::: THE' MOBEMN 'SYNTHESIS;,: -:.; :

Ascent. Rist-cousinship implies common grandparents, second-
coiBiiisMp ' C 0 II 13 I 10 E great'-graiidpaxenis., and so on. ut m
taxonomic group-relationships, descent may play a blurred or
incomplete part, or even no part at all. To take an obvious
example, numerous wild plants have wHte-flowered vaneties
in nature; but all the members of “var. alba” in bluebells no
more constitute a single group with common descent than do
all the albinos in human beings. Wherever we find sporadic
groups of variants drfiering from the type in a single mam
character, the same will apply. Many such examples are known,

both from plants and animals. t ii

Where, however, a group is characterized geographically as
well as genetically, as, for instance, with most animal subspecies,
the hypothesis of descent firom a common ancestral group is
usually tenable, especially when numerous separate genes enter
into the characterization. But even here it is not necessary.
With changed climatic or other ecological conditions, only
fprrain types and Combinations within a highly variable popu-
lation may be able to spread into new areas. They wiE then
constitute a single geographical and genetic group, but will imt
have a single common origin. This has been postulated by
Turrill for the origin of Ajuga chamaepitys from A. chia (p. 267),
and doubtless will be found to hold for many other cases as

investigators bear this possibility in mind.

Even in the coninioiier case of the diferentiation of a local
group iw sifWj the picture will be complicated by migration and
iiitercrossing with members of other groups. This may be fre-
quent, as with many contmental subspecies, or infrequent and
sporaic, as with many island subspecies; but only rarely, as on
oceanic islands, is it Mkely to be wholly absent. In any case,
with biological groups ‘xornmon ancestl:y^^ does not imply
descent from a single ancestral pair, as in human relationships;
it means the gradual modification of a more or less sharply
delimited group by the progressive substitution of some genes
for others.

The parallel with individual human relationships is particu-
larly misleading in the case of human groups, for the obvious

SPECIATION, EVOLUTION, AND TAXONOMY 399

re^on that migration, reticulate crossing, and consequent recom-
bination are more widespread in man than in any other organism.
So-called “racial types” may be mere recombinational segregants,
thrown up from a highly mixed population, without any con-
tinuity of descent through the same phenotype or genotype
from the original stock which they are held to represent; the
most abimdant types in a mixed group may well be new recom-
binations, different from any found m any of the parent stocks
from whose crossings the group arose, and so forth. The question
has been discussed in more detail by Huxley and Haddon (1935,
Chapters 3-5).

Recently, a dispute has arisen between the adherents of a
phylogenetic classification and those who maintain that the only
possible basis for taxonomy is a purely logical one, based on a
m axim u m correlation of attributes (see Gilmour, 1940, Caiman,
1940, and dhcussion in Proc. Linn. Soc. London., 152 : 234).
However, the beHevers both in a completely logical and in a
completely phylogenetic taxonomy would appear to be aiming
at ideals which are quite unattainable in practice; in addition,
both systems are m some cases not consonant wdth fact. For
instance, taxonomic practice, at any rate in larger groups among
animals , appears to base itself on the co-ordination of characters
in an organizational plan, rather than on the totality of attributes,
while a phylogenetic classification simply will not fit certain
facts of nature, such as those produced by reticulate evolution.

In practice, however, the two concepts largely coincide. They
coincide because the processes of mutation and selection distri-
bute characters among taxonomic groups in such a way as to
fulfil approximately the postulate of a maximum correlation
of attributes demanded by the upholders of a logical classification.
The more characters there are available, the greater in general
the approximation (cf. p. 371). Geographical distribution and
paleontological history are to be included among characters in this
sense. In fossd material, however (e.g. moUuscan shells), the
number of characters may be very much limited compared with
the range available to the student of Hving forms; it is probably
this wlhch accounts for many of the cases of apparent parallel

400 evolution:;'- tHE^'-MOBEIlN SYNTHESIS

.evolution, to be found in' the paleontology of e»g. -molluscs and
brachiopods,

•' When divergent groups have evolved separately for long
periods, the co-ordination of character-distribution with taxono-
mic grouping will be very close. It need not be so close, however,
when the divergence is of recent date; in this case, the chance
of parallel mutations upsetting the co-ordination is much greater.

In one respect taxonomy would appear definitely to have a
phylogenetic - basis, in that named categories are in general
monophyletic groups. Wherever the distribution of characters
contradicts the hypothesis of monophyly for a group, the taxo-
nomy demands revision; here the phylogenetic outlook can play
a constructive part in taxonomy. This generalization may break
down in regard to certain subspecies (p. 215) and species, which in
e.g. apomictic and in reticulate evolution must be delimited
purely on the basis of convenience. It also breaks down in the
case of ‘'horizontaF’ groups (e.g. genera) in paleontology, which
may be merely stages run through independently by several
lineages, and yet necessary categories for the sake of taxonomic
convenience (see also p. 409). But m regard to higher categories
the principle certainly holds.

When it comes to detailed taxonomic arrangement, however,
as opposed to taxonomic iiamhig, it is difficult to see how a
phylogenetic basis, or even a phylogenetic background, can be
found for this. As various workers have shown, the elaborate
trees and other diagrams of arrangement (relationship) proposed,
e.g. for the groups of higher plants, are largely contradictory
inter se, and must be regarded as highly speculative. Whenever
there is reasonable certainty as to arrangement— e.g. wrhen one
set of families or orders can be deduced to have a common
origin separate from tliat of others— this can and should be
represented by means of named categories, such as superfamily,
suborder, subclass, etc. Where this is not possible, the arrange-
ment (e.g. the order in which groups of a certain taxonomic
category are enumetated) should not be presumed to have any
phylogenetic meaning.

Even if we had a full knowledge of the phylogeny of, say.

SPECIATION, EVOLUTION, AND TAXONOMY 4OI

all genera and families within an order, the diagrammatic repre-
sentation of this would be exceedingly complex, and must be
held to be a “subsidiary classification” in Turrill’s sense rather
than failing within the province of taxonomy serm stricto
(Turrill’s “alpha or orthodox taxonomy”).

Giimour has pointed out that taxonomic practice was actually
little altered by the introduction of the idea of evolution and
phylogeny into biology. We must remember, however, that the
more philosophically-minded pre-Darwinian taxonomists thought
in terms of an “ideal plan” or archetype which was modified in
detail in various subgroups of a major group (see p. 391), and
that tliis is in point of fact a symbohc representation of phylogeny.

Thus, while taxonomic practice inevitably rests upon the
evaluation of characters, and while phylogenetic relationship
must always (in the absence of full paleontological data) remain
a deduction, the phylogenetic idea, whether directly, or sym-
bolically in the form of a modifiable archetype, may and often
docs aid the taxonomist in evaluating his characters and hi
frammg Iiis categories. In general, it is more correct to speak,
of a phylogenetic background for taxonomy than of a phylo-
genetic basis. And we must constantly beware of arguing in a
circle and giving independent existential value to the phylo-
genetic groupings which we have merely deduced from the
distribution of characters and structural plans in existing groups.

The possibility diat the initial separation of groups, capable
of leading on to species-formation, may in some cases be genetic
histead of ecological or geographical also introduces compli-
cations hito minor systematics. Two genetically isolated species
in the same area and habitat may remain closely similar, both
physiologically and morphologically, for long periods, whereas
two ecologically divergent species might differentiate markedly
in a much shorter period.

Again, as we have previously seen, the physiological diver-
gence found in “biological races” may become quite extensive
widiout being accompanied by more than mininial differentiation
in visible characters. It may be argued that taxonomy cannot
and should not take accoimt of time, only of divergence. But

402 BVOLUTrON MODERN SYNTHESIS

sliould it not take as much account of physiologicai as- of morpho-
logical divergence?

We have, next, the existence of polyploidy* Autopolyploids
provide one not inconsiderable difficulty; they are well isolated
as reproductive groups, but differ extremely little in visible
characters from ‘ other members of the series (though often
markedly in physiological characters and consequently in area
of distribution). But allopolyploid species arising as the result
of a cross simply do not fit into the classical framework. New
methods of denoting relationship arc needed when wc have to
take into account the convergence and union of branches as well
as dieir divergence. This difficulty is accentuated in the case of
reticulate groups (p. 353) where, as wc have noted, ordinary
taxonomic methods have already partially or completely broken
down.

Another point, of purely practical but none the less real impor-
tance, concerns the modern tendency to push the geographical-
Darwinian method of classification to a conclusion so logical
that its appheation becomes harmfiiL The battle of the ‘"splitters"^
and the ‘lumpers’* still continues, though now in respect of
subspecies instead of species. The “spHtters” wish to distinguish
as a separate subspecies, with its own trinomial designation
subject to the international rules of zoological nomenclature,
every population which can be distinguished, by however slight
a criterion, from other populations. As an example of the lengths
to which this process is already being carried, let us take a case
recently adjudicated on by the British Ornithologists’ Union,
It appears that British-breeding specimens of the common red-
shank, Tringa tetanus^ can be distinguished from their continental
relatives by a slightly darker coloration. There arc no structural
or size differences, and the colour distinction, in addition to
being slight, exists only in summer plumage. In winter pliioiagc
members of the two populations arc admittedly indistinguishable.
Yet the British form has been solemnly allotted subspccific rank.
In consequence, the continental subspecies must now, it is ruled,
be banned from the British list, since any birds shot in winter
on our shores cannot be ascribed to this form, even if wc know

SPEeiATlON, EVOLUTION, AND TAXONOMY 403

perfertly well tiiat most of them will be mutants from Europe!
Tlie subspecies cannot reacquire its British status before a Euro-
pean-ringed specimen is shot in Britain. Such decisions tend to
reduce systematics ad absurdum. This holds also for the erection
of new subspecies on the basis of being shghdy darker (e.g.
Clancey, 1938, for west Scottish birds, which are anyhow more

likely to be on a dine). 1111,

Difficulties arise in other cases where forms regarded by the
“lumpers” as subspedes vary locally. We have met with such a
case in the crows (p. 248). Hoodie and carrion crows are both
divisible into local groups with considerably better differentotion
than that of the redshanb just discussed. But if the conclusion
of the adherents of the Rassenkreis idea be sound, that hoodie
and carrion crows are themselves merely well-marked subspecies,
then we must allot “sub-subspecific” names to then locd forriis.
Apart from the practical inconvenience of any such multi-
nomial system, we should then be giving a lower systematic
rank to the local forms of crows than to those of titnuce or
wrens distinguished by approximately the same amount ot
divergence. The difficulty is real, however, and not arOiia^.
It may perhaps be avoided by using the term smispecies. This
has been proposed by Mayr (1940) for forms which can be
deduced to be geographical representatives of some other species,
but have during isolation developed morphological difterences
of the order of magnitude to be seen between undoubted speaes ;
and under the term he includes forms like the flickers (p. 250),
which hybridize in a manner precisely similar to the crows.
Taxonomically it will perhaps be best to give bmomial n^es
to such semispecies, while uniting them and their geographical
vicariants in a supraspecies, to wliich Aome name may be given
compounded from two of the binomials of the group.

Zuckerman (1940), discussing some of the defects of the
present classification of the Hominidae, points out that the desir e
to ascribe the utmost possible importance to any new fmd ot tossil
man has led to the erection of several quite unjustified genera.
He pleads for the setting up of empirical critena of cMerence
for species and genera, in the absence of that abundance of

404 evolution: THE MODERN SYNTHESIS

material which, alone could make a phylogenetic classification
really possible.

The Sabbath was made for man, not man for the Sabbath.
Similarly systematics exist for human convenience, not in the
interest of some Platonic eidos stored up in Heaven. The time
has come when we must make a decision as to the impHcations
of recent research for nomendatorial practice.

A quadrinomial system, by which genera, subgenera, species,
and subspecies are given formal names, is a useful invention for
the purposes of detailed pigeonholing. Practical convenience,
however, dictates that for the ordinary purposes of general
biology, binomialism should remain. This can be achieved if
large species of the nature of Rassenkreise, and large genera con-
taining numerous Artenkreise and other types of subgenera-, ire
used for the normal des^ation of different kinds of animals
and plants, reserving the subgenus and the subspecies for the
use of systematists or for various special purposes. The subspecies
should be more widely used than the subgenus, since different
subspecies of a species are concrete biological groups, differing
often in quite important points of physiology and be-
haviour as well as in size or other visible characters. The
common habit of splitting old-established genera into a
number of new genera, often monotypic, is frequendy an abuse
of systematic method, because an unnecessary denial of the
principle of taxonomic convenience.

Modem systematics, in so far as it is coping with geographical
divergence, must in fact recognize various fruits of its own
activities. The principle of geographical replacement has for its
taxonomic corollary not merely the degradation of many groups
from specific to subspecific rank and their grouping within
major (polytypic) species or Rassenkreise, but also the disallow-
ance of many genera and their degradation to the statm of
Artenkreise or (geographical) subgenera.

In the second place, the same principle, carried to its logical
extreme, implies that we must frequendy expect the population
of one geographical area to difier from that of another by very
small diough constant difierences. This does not, however, imply
the desirability of each such form receiving a Latin name. For

SPECIATION, EVOLUTION, AN 0 TAXONOMY 405

one the principle of practical taxonomic utility forbids

subdivision being carried too far: this is es^daUy true of names
which are subject to the rules of systematic nomenclature, and
thus enshrined for ever in an official position. For another, the
principle of character-gradients (dines) must be taken into
Lcount. Such geographical forms may prove to be merely points
on a dine. If so, then, unless a discontinuity, or at any rate a
much steepened portion of the gradient intervenes between
them, they assuredly do not deserve separate subspecific names,
but the chne as a whole should be named (p. 226).

Ihe fact that two or more dines may be operative in different
directions across the range of a species introduces yet another

compheation. i - 1-

It seems certain that systematics will have to invent subsidiary
terminologies to cope with the complexity of its data (see
Turrill, 193 8n). Genus, subgenus, spedes, and subspecies will
doubtless remain more or less universally as main categones.
The definition of genera and subgenera is often largely a matter
of convenience. Besides geographical subgenera we may also
expect other types— e.g. those of an ecological and perhaps those
of a cytologicai nature. The definition of species we have du-
cussed at length (p. 157). h is essential that, if the term is to be
retained, it should be used in a broad sense, with due regard to

practical systematic convenience. ,.11 •

Subspedes have usually been defined on a geographical bas«.
This, however, is largely due to the historical reason that the
refinements of taxonomy were most readily worked out m
vertebrates, where ecogeograpWeal divergence is the mam
factor in minor systematic ffiversity below the level of the

4o6 fvolution: the- mobeen syhthesis^:^,:,;

to the siibspecific name of the letters v and C for

geographical, ecological (induding physiological and biological),
and cytological divergence respectively would serve. In some
cases considerable geographical differentiation may occur within
genetic or biological subspecies. Here, presumably, two sub-
specific names will be required.

For specifying character-gradients (clines) it is hard to see any
fuUy satisfactory solution save the marking of them on a map.
However, a useful first approximation would be a statement
of the character they concerned and their approximate direction.
For instance, after the description of a polytypic species wliich
showed considerable geographical variation, one might add such
phrases as “Size S-N; melanin E-W from desert belt to sea,
then SW-NE"^ the increases in the character being in the geo-
graphical directions named. But the complexity of the data
might often stultify such an attempt.

When dealing with differences characterizing a regional popu-
lation, especially when tliis is geograpliically discontinuous from
neighbouring populations, regard must be had to practical
convenience. We must not erect subspecies whose diagnostic
diflferences are smaller than those of mere local groups of other
subspecies: the term subspecies should connote a moderate degree
of difference, not mere diference, however niinute (p. 402).
Practical convenience, on the other hand, makes it extremely
undesirable to introduce a new nomenclatorial category, though
the existence of such microsubspccies or microraces (Dobzhansky)
is indubitable. It would seem best for systematists in such cases
to confine themselves to descriptive statements, such as that
minor geographical forms (microsubspecies), characterized in
such-and-such a way, and perhaps denoted by a letter or number,
occur in such-and-such regions.

Another method is that suggested by Turriil for designating
varieties by combinations of letters according to the combina-
tions of characters which they exhibit. This will not be of much
service when variation chiefly takes the form of dines, but
will be useful wherever sharply-contrasting characters arc
involved, and especially so where hybridization has been at

1. a. Geograpliiral genus I f (Rensch’s ^rtenfereis).

b. Geographical sub- j- Consisting of species showing

genus J geographical (or ecological)

replacement.

2. Supraspecies Consisting partly of subspecies,

partly of semispecies or full species,
aU showing geographical (or eco-
logical) replacement.

So far geographical replacement is the only basis known for
categories i and 2, but we may prophesy that ecological replace-
ment will be detected as a basis for such categories, in insects
at least.

3. Species.

Differentiated into numerous spa-
tially co-existent ecotypes or other
sharply contrasted forms.

Differentiated into subspecies
showing geographical or ecological
replacement, or into forms vpth
diferent chromosome -number;
the subspecies may fall into dines.

c. Monotypic (mono- Not difierentiated into subspedes

morphic) (or into an array of well-marked

and co-existent ecotypes).

4. Semispecies On the borderline between suh-

spedes and species.

mori

408 EVOtUTION

5. Clines

6 . Chromosome-raccs

7. Siihspecies

8. Microsubspcdes

9. Apomict strains (clones).

Natural groups, in tlie sense here employed, have a geo-
graphical distribution qua groups and are either self-perpetuating
or have dearly been recently derived from a self-perpetuating
group. Phases, forms, and sporadic mutants are not natural
groups in this sense, nor are ecotypes. If a phase or an ecotype
becomes the only form in a given area, and persists there, it
ipso facto merits subspedfic rank. The word variety has been
used in so many senses that it should be dropped. If a general
term is required for any variant form, parmorph may serve. The
nomenclature of hybrids is discussed by Allan (1940), and the
taxonomy of cultivated plants by Vavilov (1940).

In paleontology, many difficulties arise. A technical difficulty
arises from the fact that the paleontological taxonomist is con-
jSned to fewer characters, since soft parts are not available. This
becomes acute, e.g. in many molluscs, though it is not serious
in such forms as mammals. Some paleontologists arrive at con-
clusions which do not square with the experience of taxonomists
who have the advantage of dealing with living material. Thus
Macfadyen (1940) * describing Liassic Foraminifera, writes of the
Lagenidae: “in this family there appears to be wide variation
within some of the groups, where neither ‘spedes’ nor even
‘genera’ are sharply defined.” In view of what we have pre-
viously said as to the biological reaUty of spedes, it is probable
that such conclmions derive from the inevitable difficulties of
die material (see also Macfadyen, 1941).

: THE MODERN SYNTHESIS

To be given Latin names when they
areconsiderable and continuous and
not differentiated into subspedes.

Differing in chromosome-number,
usuallyby wholegenomes; tobede-
signated by the ploidy (sn ; 4n — 3 ;
etc.) after the specific name.

SPECIATION, EVOLUTION, AND TAXONOMY 4O9

A more fundamental difficulty is the fact that he must consider
the dimension of time as weE as of space. ParaEel evolution is
a real phenomenon, but in many fossE groups its apparent
extent is exaggerated by this paucity of taxonomic characters.
Wherever paraEel evolution occurs in a group, two types of
classification are possible — ^by vertical lineages, along the time-
dimension, and by stages run through by several lineages, cutting
across the time-dimension (see e.g. Arkell, 1933; W. D. Lang,
1938). It is often advisable to give generic names to such hori-
zontal stages. It has been maintained that such “horizontal”
genera are purely artificial; but as E. I. White pointed out in a
recent discussion (unpublished) at the Zoological Society, this
is not the case; granted the ocurrence of paraEel evolution,
horizontal st^es are inevitable facts of nature. It thus becomes
necessary to introduce a double terminology, vertical as weU as
horizontal. The simplest convention would be to apply generic
names to horizontal stages and to introduce a subsidiary ter-
minology for lineages; but the details must clearly be left to the
paleontologists themselves — ^with the one proviso that they work
out a simple and agreed system. (See ArkeE and Moy-Thomas,

1940.)

Many paleontologists (see e.g. discussion in Swinnerton, 1940)
give binomial names to so-caEed “morphological species” which
are without doubt only extreme variant types arbitrarily selected
from the assemblage provided by a variable true species. This is
an unfortunate misuse of taxonomic terminology: some other
method of naming such forms should certainly be devised.

Undoubtedly the most important result of modem research
in and bearing upon systematics is that species may originate
by numerous and quite different methods, which faU under three
main heads: the geographical, the ecological (in the broad sense) j
and the genetic (cytogenetic). The degrees of morphological
divergence and intersterility between related forms vary gready
according to the method of divergence w'hich has been pursued.

Faced with the abundance of ne>y facts, we must acknowledge
that some new step in taxonomic practice is due. Two major
improvements in the methodology of systematics have been
efieaed in the past. The first was the substitution of the Lumaean

'410 ' / . , evolution: '/THE MODERN SyNTHESIS; ■

system of binomial nomenclature for tHe earlier method in which
nomenclature was confused with description. The second was
the introduction of trinomialism to cope with the data of geo-
graphical distribution. It is safe to prophesy that the next decade
or so '.will sec a third phase of major improvement. This will
involve the introduction of some method, concerned largely with
subsidiary terminologies, by which, while the principle of
taxonomic convenience is still given due weight in the main
terminology, the cytogenetic and ecological data of systematics,
and the facts concerning actual or potential interfertility, can
be adequately described and discussed. It will also involve the
reduction of taxonomic difierences to metrical form. The impor-
tance of this has been ably urged by Richards {i 93 ^)» who also
makes numerous practical suggestions. A few decades hence it
will, we may prophesy, be regarded as necessary taxonomic
routine to give the mean measurements, with their standard
deviations, of at least five or six standard characters, as part of
the description of a new form. The characters would vary from
group to group, but could readily be standardized for each
group. Leitch (1940) stresses the importance of such metliods
in psJeontology, and points out that certain assemblages can be
characterized by their degree and type of variability. Equally
important arc accurate methods for the quantitative study of the
numbers and properties of populations; sec references in Timofeeff-
Ressovsky (1940), Dowdeswell, Fisher and Ford (1940), Spencer
(1940), and Dobzhansky {1940).

It has been customary to distinguish sharply between artificial
and natural classification. But the ‘natural classification’^ at
which post-Darwinian biology has aimed is itself in certain ways
artificial. For one thing it represents an unattainable ideal. And for
another it assumes— what we now can perceive to be erroneous—
tha^ the only natural method of classification is one based on
naive and pre-mendelian ideas of relationship taken over from
human genealogy and applied to groups instead of to individuals.
Furthermore, it has unconsciously accepted certain implications
of the Aristotelian method of classifying things into genus and
species, implications which are of philosophical rather than

SPECIATION, EVOLUTION, AND TAXONOMY 4II

sdentijBc import and based on a priori logic rather than on
empirical fact. The most important of such implications is a
tendency to accept the discreteness and fbdty of separate species
(and subspecies) at more than their face value.

The new classificatory systems that are destined to arise will
be more natural, in the sense of more truly reflecting nature.
They Avill provide us with a picture of the diversification of life
as it actu^y exists, and sometimes as it has actually occurred.
They will give due weight to gradients of change, their different
directions, and their variations in steepness. They will help us
to think in terms of genes and their distribution as well as in
those of individuals. As regards the units of the taxonomist, we
shall cease to regard them as so absolute or so necessarily distinct.
We shall begin by thinking of life as a unity, into whose con-
tinuum discontinuities have been introduced. Some of these are
partial, of various degrees of completeness, while the complete
gaps are of various widths. Further, the discontinuities are of
various origins. Some are imposed by geographical causes which
are, biologically speaking, accidents. Others are the outcome of
ecological specialization, and are then often accentuated by
selection. Still others are the by-products of the working of the
physical machinery of heredity, the chromosomes, their division
md meiotic reduction. Some discontinuities arise gradually, others
abruptly. Some are the accidental outcome of isolation, others
the consequence of mere divergence, while still others have been
selectively involved so that related groups may be more effectively
kept from interbreeding.

The. new taxonomy, with the aid of its subsidiary termino-
logies and its quantitative measurements, will seek to portray
this many-sided reality. The picture will inevitably be less simple,
but it will be more true to nature. The origin of species is largely
irrelevant to the large-scale movements of evolution. But, through
taxonomy, it will be perceived as a complex and multiple pro-
cess, responsible for much of that amazing variety of life which
at one and the same time attracts and bewilders the biologist.

CHA3?XER 8

ADAPTATION AND SELECTION

1. The omnipresence of adaptation . . p. 412

2. Adaptation and function; types and examples of adaptation p. 417

3. Regularities of adaptation ........... p. 43*^

4. Adaptation as a relative concept p. 43^

5. Preadaptation p. 449

6. The origin of adaptations: the inadequacy of Lamarckism p, 457

7. The origin of adaptations: natural selection . . . . , p. 466

S. Adaptations and seleaion not necessarily beneficial to the

species. p. 47^

I. THE OMNIPRESENCE OF ADAPTATION

We next come to the origin of adaptations. It has been for
some years the fashion among certain schools of biological
thought to decry the study or even to deny the fact of adaptation.
Its alleged teleological flavour is supposed to debar it from
orthodox scientific consideration, and its study is assumed to
prevent the biologist from paying attention to his proper busi-
ness of mechanistic analysis. Both these strictures are unjustified.
It was one of the great merits of Darwin himself to show that the
purposiveness of organic structure and function was apparent
only. The teleology of adaptation is a pseudo-teleology, capable
of being accounted for on good meclianistic principles, without
the intervention of purpose, conscious or subconscious, either on
the part of the organism or of any outside power. And to the
second objection, the answer is that since adaptations are facts,
it is the business of biologists to study them. If a biologist thinks
that he has exhausted the study of a structure or a function
merely by showing its adaptive advantage, he is a bad biologist;
but so is he who thinks he has done so merely by giving a
mechanistic account of its present condition and its embryo-
logical development. The truth is of course that every biolqgical
problem has its evolutionary as well as its immediate aspect, ics

ADAPTATION AND SELECTION 413

functional meaning as well as its meclianistic basis; and both
need to be studied.

Adaptation, in point of fact, is onmipresent. The field worker
rightly laughs at the disbelievers in the adaptive significance of
mimetic or protective coloration or of threat behaviour. I have
been deceived in Africa by the resemblance of a mimetic spider
to the ants with which it associates*; have spent vain hours on
a Surrey common searching for a nightjar’s nest, so perfect was
the bird’s cryptic coloration, before stumbling accidentally
upon it; have nearly fallen out of a tree when a wryneck on its
eggs simulated a hissing snake. That the examples of protective
coloration, afforded by the leaf-msect, the woodcock, the dab,
or the twig-hke larvae of geometrid moths, should be hackneyed
is no argument against their biological validity. Nor does the
disbelief of certain laboratory mechanists in warning coloration
and other aposematic characters prevent chicks from associating
the black and yellow of cinnabar caterpillars with nauseousness,
or hinder human beings from paying attention to the rattle of
a rattlesnake. The biologist who discovers by comparative study
that the metabolism and respiratory pigments of animals are
closely adjusted to their mode of life is not likely to imagine
that the correspondence is fortuitous. The physiologist who
unravels the postural reflexes of a bird or investigates the chemical
regulation of respiration-rate is not likely to dismiss organic
function as non-adaptive; the naturalist who notes the constant
correspondence between structure and inborn behaviour on the
one hand and environment and way of life on the other one
has only to think of sloth and owl, anteatcr and flamingo, angler
fish and whalebone whale — must believe either in purposive
creation or in adaptive evolution; the evolutionary biologist
who finds that the rise of each new dominant group in turn is
associated witli some basic improvement in organic mechanism,
be it in the shelled egg, or warm blood, or placental reproduc-
tion, will have to admit that adaptation has been all-important
in evolutionary progress.

It is perhaps unfortunate that the study of adaptations has

* For a coloured figure of a spider mimicking an ant, see Donisthorpe (1940}

414 evolution: the modebn synthesis

been so closely associated with highly specialized and striking
cases of the “wonders of nature” type, such as the resemblance
of a butterfly to a dead leaf complete with mould-spots and
imitation holes, or the almost fantastic contrivances of certain
orchids which secure insect-pollination. For this tends to distract
attention from the bedrock fact that some degree of adaptation
is omnipresent in life, and that this fact demands an evolutionary
explanation.

However, in his recent very striking book Cott (1940) has
shown that concealing and revealing coloration, when properly
investigated, remain the paradigm of adaptive studies, and has
thoroughly turned the tables on captious objectors. Such critics
of the theories of protective coloration and mimicry have been
in the habit of dismissing them as pure fantasies or armchair
speculations. A. F. Shull (1936), for instance, goes so far as to
state that the theories of aggressive and alluring resemblance
“must probably be set down as products of fancy belonging to
uncritical times” (p. 175), and concludes (p. 212) that “if the
doctrine [of natural selection] can emerge minus its sexual
selection, its warning colours, its mimicry, and its signal colours,
the reaction over the end of the century will have been a distinct
advantage”! The array of facts presented by Cott shows that
it is these objections which deserve the designation of “arm-
chair”: it is the field naturalist and the experimental biologist who
provide the facts from which the theories arc educed. Cott (and
see Carpenter, 1939) also summarizes the numerous experiments
and observations which demonstrate the reality of selection operat-
it^ in nature in favour of cryptic or aposematic coloration. He
also points out the irrelevance ofthe criticisms of McAtee (1932).

In addition, Cott analyses the features of pattern by which
illusions of various sorts, whether for decrease or increase of
conspicuousness, can be created, and then demonstrates their
existehce in nature. The particular method employed will be
related to the type of habitat occupied. Thus inconspicuousness
of the flat wing of a butterfly in low rough herbage is generally
obtained by a false illusion of relief; the obliteration of sharp
outline in a tangle of vegetation tends to be achieved by counter-

ADAPTATION AND SELECTION 415

shading together with ruptive markings, whereas with forms
which must expose themselves on bark it involves arrangements
for preventing marginal shadows, often coupled with an actual
irregularity of the outline itself, achieved by irregular outgrowths.
Most convmcing are special correlations of pattern with unusual
positions : an excellent example is the reversed countershading of
sphingid caterpillars which feed at night, but rest in an inverted
position by day, and of the peculiar Nile catfish Synodontis
hatensoda, which swims upside-down (see Norman, 1931, pp. 29,
227).

It is interesting that Suffert (1932, 1935), as the result of
intensive studies pursued without knowledge of Cott’s work,
arrived at similar conclusions. Three recent independent observers
may also be quoted. Comes (1937) cites the moth Vemisia veni-
culata, which lives on a particular tree-hly. Its wings are marked
with lines running at right angles to the body; and at night it
invariably orientates itself across a dead leaf, so that its markings
coincide with the conspicuous longitudinal lines on the leaf.
Its antennae, which would destroy the resemblance if visible, are
tucked out of sight under the fore-wings. When disturbed it
settles down again, “after a few compass-like vacillations”, in a
similar position.

Again, W. W. A. Phillips (1940) describes the nest of Hemipus
picatus leggei, a shrike from Ceylon. The bird nests on bare Hmbs
of trees. The nest is not only camouflaged with lichens and
bark flakes, but its sides are built down flush with the branch so
as to resemble a knot. Finally, the fledgling young, so long as
the parents are away, sit motionless facing each other with eyes
half-closed and beaks pointing upwards and nearly touching in
the centre. Their coloration is a mottled drab and blackish grey,
so that they are almost invisible, even when nearly fledged.
From a distance of little more than 12 feet the nest with the young
bears a inost remarkable likeness to a snag left on the upper side
of a branch through the breaking off short of a smaller branch
just beyond its Junction with the major stem. The upward-
pointing beaks help to heighten this similaiity; they represent
the sharp-angled fracture left at the top of the stump. This

4.16 EVOtUTION: THE MODERN SYNTHESIS

cxamplcmay be compared with the protective coloration-a//H-
attitude of the brooding nighgar Nyctibius griseus (see Cott, 1940,
Fig. 74), but is almost more remarkable as involving a co-
operative attitude on the part of several birds.

Finally, Holmes (1940J describes the unique case of the common
cuttlefish, Sepia officinalis, wliich can change its colour and pattern
within the space of a second. By this means, it can draw on an
amazingly varied repertoire of protective devices, including con-
cealment by means of obliterative shading, close environmental
resemblance, striking ruptive patterns, and flash patterns which
bewilder an enemy by their extremely rapid sequence and great
difference from each other, and also the scaring of enemies away
by conspicuous threat patterns. Any particular one of these Will
be adopted according to circumstances. In addition, it employs
special epigamic. stimulative patterns in courtship. Related
cephalopods do not show this multiform adaptation, which can
be related to the particular habits of the species.

In regard to mimicry, the detailed following by the mimic
of the pattern of the model, as the latter changes geographically
from subspecies to subspecies, constitutes a beautiful case of
detailed adaptation (see e.g. Poulton, 1925, PL D). This pheno-
menon is not due to any direct or indirect cft'cct of climate. (Sec
also p. 102; Carpenter and Ford, 1933).

As Cott rightly says, physiologists and anatomists do not
dispute as to whether a wing is or is not adapted to flying:
they set themselves to discover the extent to which, and the
precise method by which, it is adapted to that function. Colour
and pattern in this respect fall into line with any other functional
attribute of organisms.

Actually, in view of the remarkable studies of particular kinds
of adaptations made in die latter half of the nineteenth century*
the increduhty shown by a certain school of modem biologists
appears very remarkable. Thus, to take only one example — the
various adaptations concerned with cross- and self-pollination in
higher plants — ^wc have the intensive work of Darwin (1877)
on orchids, and the cxliaustive survey, largely original, by
Kemcr (Kcmcr and Oliver, 1902). After reading Kerner’s

ADAPTATION AND SELECTION

417

account of the devices for securing cross-poUinadon, and those
equally remarkable ones for securing self-pollination, the two
often co-existent in the same flower as what the Germans call
doppelte Sichermg, there would seem to be no room left for
scepticism on this point. And if on one point, why on others i
However, Cott’s book deserves special attention, since it
takes account of all the objections, theoretical, factual, and
methodological, raised by the sceptics of the early twentieth
century.

T. H. Morgan (1932, p. 115), in reviewing the subject, makes
the following pertinent remarks. “A fact of some interest becomes
apparent at once, namely, that what are usually cited as adapta-
tions are instances in which a species shows some unusual type
of structure, i.e. one in which it departs from most of the other
species in the group. In other words, it is the exceptional that is
often referred to as a typical case of adaptation. The reason for
this is apparent. The exceptions stand out conspicuously as
specialties for some particular situation. Nevertheless, a moment’s
thought should show that the general problem of adaptation is
not to be found so much in these particular occasional departures
as in the totahty of the relations of the organism to its environ-
ment, which makes the perpetuation of the individual and of
the species possible. The extreme cases catch our attention, and
their special relation is sometimes more easily seen, or guessed
at, than the more subtle physiological processes that make all
Hfe possible.”

2. ADAPTATION AND FUNCTION; TYPES AND EXAMPLES
OF ADAPTATION

Adaptation and function arc two aspects of one problem. We
may amplify this statement by reminding ourselves diat die
problem of adaptation is merely the problem of functional
efficiency seen from a slightly different angle. There are certain
basic functions, such as assimilation, reproduction, and reactivity,
which arc inherent in the nature of living matter, and can thus

4i8 eyoiution:; the modern synthesis

hardly be called adaptations. But any of them can be specialized
or improved in various -ways during evolution to meet the
needs of the organism. The fact, for instance, that our gastric
glands begin to secrete when our nose or eyes are stimulated by
the siiicU or sight of food, is an adaptation concemed with
assmiilation, just 'as is the elaborate structural raminating mechan-
ism of the oxen and their allies.

The distinction between basic property and superposed adapta-
tion may be well brought out by a historical example. Weismann
considered the property of regeneration to be a special adapta-
tion, acquired during the course of evolution by such animals
as were especially exposed to loss of limbs or other damage.
Experiment, however, failed to confirm this conclusion: for
instance, Morgan found that the abdominal appendages of
hermit-crabs, though normally protected by the hard molluscan
shell inhabited by the animal, regenerate just as readily as the
exposed big claws or walking legs. Further, on general grounds
it became more and more obvious that regeneration depended
essentially on the basic capacity of living matter for reproduc-
tion and growth. Regeneration is to-day universally looked upon
as one aspect of an inherent quahty of life, and the chief problem
set by it to biology is not how to account for its presence in
lower forms, but how to explain its restriction and absence in
higher types.

Frequently associated with regeneration, however, is the faculty
of autotomy or scif-mutilation, whereby an animal detaches a
limb, like a lobster, or a tail, like a lizard, sacrificing a part rather
than risk the whole. In most cases autotomy takes place at
definite spots. The higher crustacca have special breaking-joints
which enable them to throw off their claws and legs easily and
with hardly any loss of blood; similar but less rigidiy-prcdctcr-
luincd breaking-joints occur in lizards’ tails. It appears quite
clear that whereas the regeneration of a lobster’s claw is a survival
of a basic property of life, its autotomy mechanism is a more
special adaptation — ^to the risk of the animal being unable to
escape if it is seized by the claw, and to the dangers of loss of
blood if the exposed claw is damaged.

la addition to the basic functions, others may arise in the
course of evolution to meet the needs of the particular type.
Thus active locomotion is absent in most plants; and colour and
pattern can only play an adaptive role in relation to higher
animals with their elaborate sense-organs.

From the point of view of selection, adaptations fall into two
categories — ^those of preadaptations fitting an organism for a
different environment or mode of life from the outset (p. 449), and
adaptations in the ordinary sense, gradually evolved within the
normal environment, whether stable or changing.

A biological classification shows that adaptations fall into a
few main groups. In the first place there are adaptations to the
inorganic environment. Some of these, like the temperature-
adaptation of local races in Drosqjhila (p. 191), or in frogs as
described by Witschi (p. 235), or of tropical as against arctic
organisms, may be of a general physiological nature, unrevealed
in any structural peculiarity. Others, like the climbing and para-
chuting habits of animals in tropical forests, or the black or red
colour and the luminosity of deep-sea animals, are more special-
ized. Hesse and others (1937) in their Ecological Animal Geography
have produced an imposing array of the general types or regu-
larities of adaptation imposed upon various types of fauna by
the pecuharities of their inorganic environment. Frequently we
can deduce an animal’s mode of life and habitat from the struc-
tural adaptation which it possesses. Occasionally we may be
puzzled, but find that fuUer knowledge solves the puzzle. Thus
the association of prehensile tails, indicating arboreal life, and
fossorial forefeet indicating burrowing habits, in some of the
South American aiiteaters, appears a paradox, until we remember
that the fossorial claws are needed to open up the nests of tree-
termites (see Emerson, 1939, p. 293).

Next wc may take adaptations concerning the organic environ-
ment — covering the functions of protection against enemies, the
pursuit of prey, reaction against infectious disease and parasites,
and the Hke. These arc essentially mterspccific. We also find
intraspecific adaptations, concerned with competition or co-
operation between individuals of the same species, e.g. the rapid

420

EVOLUTION': : THE MODERN SYNTHESIS ,

growth of inany plant seedlings, and the recognition marks of
gregarious mammals and birds.

Finally liere are adaptations of a more internal nature, con-
cerned with improvement in functions such as digestion or
excretion; or witli general co-ordination, whether by nervous
or endocrine means; or with the regulation of the internal
environment. Reproduction may also be considered in this
category. As examples of these various internal adaptations we
may take the adaptation of the form of the d^estive tract and
the kinds and quantities of enzymes produced by it to the type
of food normally eaten; in nervous co-ordination, we need
only think of the inborn mechanism whereby every time a
limb-muscle- is stimulated to action, its normal antagonist is
inhibited and relaxed, enabling the contraction of the other to
be more effective; in internal regulation we may take the
astonishingly delicate mechanism whereby the acidity of the
blood is kept constant in higher mammals; in reproduction we
need go no further than the human species and reflect on the
mutual reaction between early embryo and uterus by which
the elaborately-organized placenta is produced.

These various classes of adaptations of course overlap and
intergrade. None the less, an enumeration of them is useful in
reminding ourselves that adaptations are nothing else than
arrangements subserving specialized functions, adjusted to the
needs and the mode of life of the species or type. Most adapta-
tions belonging to our fint two categories subserve functions
usually called ecological, while the functions of most of those
in the last group arc physiological. The concept of function
has for so long been the preserve of physiology in the restricted
sense that we arc apt to forget that ecological function is of
equal importance to the species.

Our enumeration will also serve as a reminder of the omni-
presence of adaptation. Adaptation cannot but be universal
among organisms, and every organism cannot be other than a
bundle of adaptations, more or less detailed and efficient, co-
ordinated in greater or lesser degree.

On the other hand, adaptations subserving different functions

ADAPTATION AND SELECTION 431

may be mutually destructive, e.g. high specialization for sexual
display is antagonistic to cryptic resembl^ce. In such cases, the
balance between the opposing tendencies will vary in a very
instructive way according to the ecology of the species (see p. 426).
Artificial selection, as so often, provides valuable parallels. Thus
some breeds of dogs, such as bulldogs and St. Bernards, owe
their appearance to genes which are on the verge of inducing
lethality, and can only be retained by selection of compensatory
modifiers (see p. 71). Again, very high milk-producing capacity,
rapidity of growth, or extreme conformation for meat purposes
in cattle, pigs, etc., may be close to the Hmit of physiological
possibihties; in inferior environments (backward tropical regions)
animals of this type cannot maintain themselves, so strong is
counter-selection.

From the inexhaustible array of possible examples, we may
select a few wHch have been subjected to quantitative analysis,
which are unfamiHar or striking, or are of particular importance
for evolutionary theory.

A. H. Miller (1937) has analysed in detail tlie structural pecu-
liarities of the Hawaiian goose {Nesochen sandvicensis). This is
an endemic of the Sandwich Islands, and exhibits specialization
towards a non-aquatic running and chmbing habit, with restric-
tion of flying power and absence of migration. Its liabitat is
arid, and not only does it appear never to enter water, in the
wild state, but never to drink it except in the form of dew.
In correlation with its specialized habits, the webs of the feet are
reduced, the legs increased markedly in relative size; a number
of muscle and tendon characters (quite different from those
prominent in forms specialized for swimming) promote walking
and running ability, while the long and flexible toes, with the
large plantar pads, help it to climb among the steep irregular lava-
flows; the wings and sternum, on the otlier hand, are definitely
reduced. This example is of course much less striking tlian many
classical cases, such as that of the giraffe or the mole, but it illus-
trates the general adaptive correlation of structure with habit, so
clearly set forth by Boker (1934). Similarly die thrasliers {Toxo-
stoma) are adapted to digging (Engels, 1940).

422 EVOLUTION : THE MODERN SYNTHESIS

From a rather difierent point of view the exhaustive work of
Sick (1937) is worth mentioning. His detailed analysis of feather-
structure in flying birds demonstrates that feathers exhibit adap-
tations for efficiency in flight down to the smallest and most
unexpected details of structure and intercorrelation.

Desert animals show interesting behavioural adaptations
^linst hi^ winds (Buxton, 1923, p. no). Thus various desert
butterflies spend most of their active life flying about inside quite
small bushes, in order to avoid being blown away; and various
desert birds, like Clot-bey’s lark (Rhamphocorys clot-bey), fortify
the rim of their nest with ramparts of pebbles.

Our next set of examples concerns adaptations for the per-
formance of a function overlooked by most biologists, that of
toilet in mammals, on which Wood-Jones (i939l») has just
published a valuable essay.

The most interesting cases are concerned with the care of die
coat. Ungulates lack special structural adaptations for this func-
tion, and substitute the crude method of the rubbing-post, com-
bined with a very restricted application of the tongue, and m
some instances with the almost equally crude use of horns or
anders. In Equidae the subcutaneous muscle-sheet is highly
developed so as to be capable of strong twitching; this, while
mainly directed against flies, has a subsidiary toilet function.

In various manunals the tongue is the chief toilet organ. Its
greatest specialization for this function is seen in the Felidae,
among which it is much rougher than in other mammals.
Wood-Jones seems to be correct in maintaining that this rough-
ness has been evolved primarily as a brush-and-comb. In regard
to behavioural toilet adaptations also the cats are spedali^:
they are the only animals to lick their paws and use them to
reach parts of the head not accessible to the tongue. Other
or^ns that may show special toilet adaptations are the teeth
and the feet. The most remarkable of these are the procumbent
lower incisors and canines of the lemurs. These have all become
strangely modified both in shape and position, so that they
constitute a' most efficient six-toothed comb, the downward
strokes of which are well suited for dealing with the animals’

ADAPTATION AND SELECTION 433

thick woolly fur. Further, just as combs need cleaning, so do
these teeth; this secondary toilet function is carried out by an
abnormally developed sublingua. It should be noted that the
development of teeth as toilet adaptations in lemurs is correlated
with the almost complete substitution of nails for claws on
their digits.

The unrelated “flying lemur”, Gdleopithecus, and die bats are
also precluded, though in another way, from the full use of
their feet as toilet organs; and Wood-Jones points out that they,
too, have lower front teeth which appear to be adapted as combs,
though in a different way from the lemurs’. In Galeopithecus
there is also a secondary toilet organ, in the shape of the serrated
front edge of the tongue, which acts as a tooth-brush for the
pectinated teeth. The toilet function of the special teeth has not
been observed here as it has in the lemurs, but may with reason-
able certainty be deduced.

In marsupials, Wood-Jones has observed that the polyproto-
donts use their incisors as combs, so that the small size, large
number, structure {and in some cases position) of these may be
regarded as toilet adaptations, though the hdnd feet are also
employed (as we employ both brush and comb). The few large
incisors of the diprotodonts, on the other hand, are ill-suited
for this purpose, and not employed in the toilet. Here, the
united but much reduced syndactylous digits of the hind feet
appear to be of use solely as todet instruments. As with the
teeth of the polyprotodonts, their size and shape are correlated
with die length and type of fur with which they have to deal.
The bandicoots (Peramelidae) appear to be an exception, since
they are polyprotodont but syndactylous. But observation shows
that the shape of their teeth is not adapted to acting as a comb,
so that the exception proves the rule.

Dusting instincts are among the important toilet adaptations,
and may restrict habitat (e.g. in Dipodomys: Dale, 1939).

Among the other cases cited by Wood-Jones we may mention
the special bristly brushes on the feet of certain bats.* But enough

* Actually the most elaborate of all structuraltoilet adaptations are found in
higher insects, such as ants and bees.

424 hvolution: the MOREKN SYNTIII'.SIS

has been said to sliow die common diaracteristic of a particular
type of adaptation. It is concerned with a tunction : the function
may be carried out by different organs or combinations of organs
in different forms; and the organs concerned show different
degrees of structural modification correlated with efficiency in
carrying out the function.

A word may here be devoted to the nest sanitation of birds,
as this is a good illustration of an adaptation with two points of
special interest — it is transitory, but unlike other transitory adap-
tations such as the foetal membranes of amniotes, the egg-
tooth of birds, or the larval structure of ecliinoderms, it is
wholly or mainly a matter of behaviour. In almost all birds, the
nest-cup shows a degree of cleanliness which is astonishing until
one reflects on the impossibility of rearing a brood of nestlings
in their own filth. Tins cleanliness is secured in various ways,
hi some forms, such as birds of prey in the later nestling stage,
the young defecate only after backing up to the nest-rim; in
these, specially developed muscles ensure that the faeces are
projected well clear of die nest. In most passerines (and some
other forms), the droppings are encased in a gelatinous sac
secreted by the nestling’s intestine. This makes it easy for the
parait bird to handle the droppings, which are eidier eaten or
carried away to a distance. In some eases they are eaten while
the nestlings are smaH, but removed when they grow larger,
and in still odicr c^s (e.g. starling, wren, swallow) a third stage
is added in which the young evacuate backwards, clear of the
nest. In some woodpeckers, the parents niix the nestUngs’ excreta
with sawdust to facilitate hmdling. Young kingfishers appear to
use the innermost part of the nest-tunnel as a latrine. In various
.specie with domed nests, sudh as the willow warbler {Phyllo-
scopus trochilus), the nesdings eject their faecal saCs on to the
outer rim of the nest, outside the entrance hole and to the side
of it, whence they arc removed by the parents. But perhaps the
most interesting fact is that in many species the nestlings will not
defecate until the parent taps the cloaca with its beak, often
awaiting rcHeffor long periods with upturned postcrior ! Ail these
adaptations cease to operate, whether in parents or nestlings, as

AJDAPTATION AND SELECTION 425

soon as the young birds leave the nest (Blair and Tucker, 1941).

The delicacy of transitory adaptation is shown by the larvai
jaws of the parasitoid Glypta haesitator (Cameron, 1938). These
are feebly developed in the second and third instars, when only
fluid food is taken, but are powerful in the first, when they are
needed for eclosion, and the fourth, when drey are required for
feedii^ on solid food and for eating a way out of the host.

In conclusion, we may mention some cases of adaptation for
display among birds. Stonor (1936, 1938, 1940), gives a detailed
analysis for the birds of paradise (Paradiseidac) and shows
conclusively that the remarkable variety of display structures
and the equally remarkable variety of display attitudes found
in the family are invariably combined to produce the maximum
of visual eflfect. Two examples must suffice. The rifle-bird,
Epimachus (Ptilorhis) paradisea, has a display quite unlike that of
any other member of the family, in which the wings are spread
in butterfly fashion; and the effect is enhanced by the broadening
of some of the wing-feathen, resulting in a broader and more
conspicuous wing. Again, the lesser superb bird of paradise
{Lophorim superba), has two small patches of specially iridescent
feadhers on the head. For display, these are erected in such a way
as to catch the light and appear as brilliant false eyes.

Stonor (1940) gives an equally illuminating functional analysis
of the displays of the pheasants and their allies (Phasianidae).
We may cite one little-known example. In Bulwer’s pheasant
{Lobiophasis bulweri), the hinder feathers of the compressed tail
are stiff and project downwards. In display, they are rapidly
drawn through the dead leaves of the forest floor, and enhance
the striking visual efiect by means of sound (Heinroth, 1938).

Among the herons, I have myself studied the display of
the Louisiana heron {Hydranassa tricolor) and the lesser egret
{Egretta Both have a crest, somewhat lengthened neck-

feathers and special feathery aigrette plumes on the back. How-
ever, the latter are much more highly developed in the egret,
the crest and neck-feathers in the heron. And m correlation with
this, the egret in display bends down so as to render the fan of
filmy aigrettes conspicuous, while the heron erects its head and

426 evolution: the modern synthesis

neck, and the visual e£Eect of the display depends mainly on the
crest and much-bristled neck-plumes (Huxley, i9Z}h).

Conspicuousness is an essential of display: but this function
runs counter to the need for concealment. The reconciliation of
these opposing selective tendencies is effected in various inter-
esting ways (sec Huxley, 1938c). Where the need for visual
concealment is least, as in dense forest, selection for conspicuous-
ncss can have full play. It is certainly no coincidence tliat the
most brilliant secondary sexual characters arc found in forest
forms such as birds of paradise, peacock, most pheasants, trogons,
many humming-birds, etc. (Stonor, 1940). Where the need for
more concealment is greatest, as in defenceless birds of open or
relatively open country, display-coloration may be wholly absent,
as in die skylark (Ahuda arvetisis), and visual stimulation must be
effected solely by striking behaviour. In other cases, as with the
prairie chicken {Tympamchus cupido) of the American prairies,
a compromise is effected by which the display characters are
normaUy invisible and the bird is markedly cryptic, but become
strikingly conspicuous (in this case by expanding of concealed
patches of bare yellow skin on either side of the neck, until they
look like half-oranges) during die display itself. The great bustard
{Otis tarda) of the European plains is another striking example,
which, by inflating an enormous throat-pouch and everting the
wings to show normally concealed white feathers, transforms
itself from an inconspicuous to a highly conspicuous object
during its display.

Finally, the difference in reproductive habits in birds makes it
possible to calculate the differing selective advantage diat accrues
from success in mating (Huxley, 1938a and h). We may distin-
guish fractional, unitary, and multiple reproductive advantage.
Fractional reproductive advant^e is provided by stimulative
characters whose effect is merely to raise the reproductive effi-
ciency of a single mate. Unitary reproductive advantage accrues
to monogamous forms from characters adapted to securing a
mate in the first instance: the male bird either secures a mate
and reproduces, or docs not do so and fails to reproduce. And
multiple reproductive advantage accrues in polygamous forms

ADAPTATXON AND SELECTION

from characters adapted to securing mates and in promiscuous
forms from those adapted to securing coition: success here
means transm itting successful characters to the ofispring of many
mates instead of only one. In correlation with these differences
in selective value, characters with a fractional advantage, Hke
display-characters in nionogamous territorial passerine birds, in.
which display occurs only after a mate has been secured, are
never, very strongly developed. But in such forms a number of
males regularly fail to secure a territory and a mate; and the
characters concerned with securing this reproductive advantage,
such as song, are striking and may appear exaggerated or “hyper-
tehc”. Finally, where multiple reproductive advantage exists,
display characters and display behaviour normally reach an
extraordinary pitch of exaggeration, as in ruff" (Machetes), pea-
cock (Pavo cristatus), various pheasants and grouse, birds of
paradise, etc., and the display-characters may even be clearly
disadvantageous to the individual in aU aspects of existence other
than the reproductive, as in the train of the peacock, the wings
of the argus pheasant (Huxley and Bond, 1942, Proc. ZooL Soc.
A.3 : 277), or the plumes of some birds of paradise (and see p. 484).

The giant panda {Ailuropoda mlanoleuca) has recently been
shown to , possess an unexpected structural adaptation to its
special feeding habits (Wood-Jones, 19394). As is well known,
this aberrant carnivore lives almost exclusively on bamboo-
shoots. In order to hold these properly while feeding, the sesa-
moid bone on the radial side of the hand has been much enlarged
and furnished with a regular articulation with the scaphoid
bone, and a muscle which normally runs to the base of the poUex
has become diverted to it. The sesamoid with its overlying
horny pad has thus become modified into an organ functioning
as an opposable thumb. The actual pollex was apparently too
specialized to be modified in this direction. Through this re-
markable adaptation the giant panda has become endowed with
dehcate grasping capacity far beyond that of any other member
of the order, though the common panda (Ailurus fulgens)
shows some modification in this direction.

As one more example of this type of adaptation we may take

428 evolution: the modern synthesis

the external ears of the nocturnal bush-babies (Galago). These
are very much enlarged, to catch and concentrate sound-vibra-
tions. They are also mobile, like the enlarged pimiae of many
other mammals, thus ensuring a considerable degree of direc-
tional hearing. Finally they (together with the ears of otlier
lorisoids, but to a greater degree) are unique in having transverse
discontinuities in the cartilages which enable them to be rapidly
folded up, dius obviating damage to the delicate piimae from
contact with branches, etc. (Osman Hill, 1940). Here we liave
three sets of modifications all subserving one adaptive function.

A recent study by Thorpe (1936) on the Hfe-Iiistory of the
chald-d Ettcyrtus infelix, parasitic on a scale insect, will serve as
an example of an unusual adaptation. On reaching its fourth
instar, the parasitic larva changes its position and becomes in-
vested with a membranous sheath produced by the host. The
sheath then becomes attached in an extraordinary manner to the
main lateral tracheal trunks of the host, in four (or six) separate
places close above the larval spiracles, in such a way that air can
pass through, and the parasite from then on respires at the expense
of its host. “The conclusion that the whole structure is an adap-
tation for the respiration of the parasite seems inescapable.”

Such “induced adaptation”, utilizing the tissues of a host
organism, is of course also found in gall-producing animals;
the galls they produce may be highly elaborate structures, clearly
adaptive in protecting and sheltering the parasite. To quote
firom Went (1940), “The complexity of the structures induced
by the gall insects is often astounding. The central part of the
gall with the insect in it may become detached after it is full
grown. Then the insect will be released from this box through
opening of a pre-formed lid. . . . The inside of the larval
chamber is often lined with cells very rich in proteins.”

Adaptation is as normal in instinct as in stracture. The host-selec-
tive instincts of parasitoids hardly ever misarry (W. R. Thompson,
1939); the specificity of such instincts is secured by utilizing a
distinctive combination of a few sensory clues (Russell, 1941). The
curious roosting instincts of the hombill Lophocews mehnoleucos
(Ranger, 1941) are adaptations to secure its nocturnal safety.

ADAPTATION AND SELECTION

429

Adaptation is just as often manifest internally as externally,
in improvement of some physiological function as in better
adjustment of some obvious external character like colour or
pattern to the environment. Thus, to take a recently investi-
gated example, the giant nerve-fibres of various cephalopods
constitute an adaptation for quick and simultaneous contraction
of the mantle to expel a jet of water (Pmnphtey and Young,
1938). In LoUgo, the size of the fibres is graded, larger fibres
being found in longer nerves; “this is apparently a further device
for securing more nearly simultaneous contraction”.

The adaptation of parasites to their hosts comprises a wide
range of physiological features, among which die degree of
vindence may be singled out here. As is well known, many
parasites are only milcfly or not at ail pathogenic to their natural
hosts, though extremely virulent when given the opportunity
of attacking “virgin” hosts, e.g. the trypanosomes of wild game
when they obtain a footing in domestic catde. While this is in
part due to an adaptive incresae of resistance on the part of the
hosts (cf. the resistance to measles, etc., of human populations
which have been long exposed to the disease, while unadapted
populations exhibit a high mortahty), it may be in part due to
the parasite developing an adaptive lower degree of virulence.
For it is obviously a disadvantage, from a survival standpoint,
for a parasite to kill its host, so that strains of too high virulence
will tend to eliminate themselves.

Adaptations to symbiosis are sometimes very striking. Thus
numerous animals are enabled to exist in wood by utilizing fungi
which break down the wood and probably also act as a source
of food for the animal. Special pockets are often produced by
the animal, in which a supply of the fungus is carried. This
occurs for instance in the larvae (probably only the females) of
the wood-wasp Sirex (Parkin, 1941); for numerous other
examples, see Buchner’s book (1928). That the special organs are
definite adaptations for ensuring a constant supply of the sym-
bionts carmot be questioned. Tridacna has remarkable adaptations
for exploiting algae (Yonge, 1936), including lenses for increasing
photosynthesis.

430 EVOLUTION* THE MODERN SYNTHESIS

Again, the comparative study of respiratory pigments and
respiratory behaviour in animals has revealed a series of respiratory
adaptations to way of life (see p. 435). Recent work on animals
with ciliary feeding has similarly revealed the existence of diverse
and elaborate adaptations adjusting the ciliary mechanism to
different modes of life. (cf. Yonge, 1938^).

The total range of diese functional devices is very large, and
(once the hypothesis of special creation is ruled out) can only
be ascribed to accurate selective adaptation. We need not
continue the list: it would be almost coterminous with the data
of comparative physiology and physiological ecology.

3 . REGULARITIES OF ADAPTATION

The perusal of such a work as Hesse, Alice, and Schmidt’s
Ecological Animal Geography (1937} shows that the study of
faunas and floras co n fined to particular habitats will invariably
reveal certain recurrent peculiarities. Sometimes these recurrent
characters are obviously, or at least pritna facie, adaptive, like tlie
coloration of desert or pelagic forms, the prevalence of special
touch-organs and of luminescence in the deep sea, webbed feet
in aquatic birds and mammals, or prehensile tails in forest-living
vertebrates. In other cases they are correlated characters in
Darwin’s sense: this applies, for instance, to some (though not
all) of the reduction in relative size of exposed parts like ears
or limbs, in subspecies or closely related species of mammals
from high latitudes (p. 213). In still other cases, their significance
is doubtful, but even then the fact of their correlation with a
particular habitat must be of some significance, and points the
way to further analysis.

We have already mentioned certain regularities of variation
iii discussing dines (pp. 21 1 seq.), and given reasons for believing
that most of them were genetic and adaptive, though the visible
characters concerned might often be only correlates of the
invisible physiological adaptations.

In other cases, we cannot be sure whether the regularities
are genetic or purely modificational. Among these wc may

ADAPTATION AND SELECTION

431:

mention the parallel variation seen in many related species of
fish with decreasing salinity (see e.g. Mobius and Heincke, 1883),
the tendency of fresh-water mussels to be more globose in larger
waters (Bah, 1922), or the increase in thickness and spinosity
of shell in the river snails of the genus I0 as one proceeds dowii-
strcaiii (Adams, 1915). One must therefore suspend judgment
as to the adaptive nature of such regularities pending experi-
mental analysis.

As illustrating genetic regularities, we may take those of desert
grasshoppers (Acrididae), as described by Uvarov (1938). This
example is perhaps specially pertinent, since Uvarov is an
opponent of all adaptational interpretations. He distinguishes
four main faunas within the major climatic habitat afforded by
desert — the dcserticolous proper, inhabiting bare open ground;
the saxicolous, inhabiting the rougher habitat provided by the
rocky slopes of low eroded desert mountains; the arbusticolous,
inhabiting the xcrophilous shrubs of many deserts; and the
graminicolous, inhabiting the perennial grasses of certain desert
plains. These four types differ markedly in body-shape. The
dcserticolous forms have a depressed body (measured on the
metathorax) with width-height ratio from 1*0 to 2-0. In saxi-
colous forms the ratio is from 0-7 to i*o, much of the height
being due to a prominent narrow dorsal crest. Arbusticolous
forms have a similar ratio, though without the crest; and
the graminicolous forms have the most compressed bodies
of all

In addition, dcserticolous forms tend to be hairy, with punc-
tured, wrinkled, or otherwise sculptured surface, and close
resemblance in colour to the soil, often coupled with flash
coloration in the hind wings and legs. Most of them arc good
fliers. In saxicolous forms, the sculpturing is niucli coarser (the
above-mentioned dorsal crest being itself an example of this),
and there is a considerably higher percentage of flightless forms.
Coloration is similar to that of the first group. Arbusticolous
forms possess “climbing legs”, which differ in their proportions
from the jumping legs of the first two types; they also exhibit
concealing coloration, which here, however, tends to be greyish-

432 evolution: THE MODERN SYNTHESIS

green. Finally, in the graminicolous forms hairiness and surface
sculpture are usu^y negligible, flash coloration is absent, and
general coloration is that of green or dry grass, frequently with
the sharply-defined light longitudinal stripes that Cott (1940)
has shown to be obliterative in grassy habitats.

In spite of Uvarov’s anti-adaptional bias, it would seem clear
that in these various faunas coloration, both general and flash,
body form, and body sculpture are all adaptive. If the form of
the legs in the arbusticolous forms can be designated by the
functional term of “climbing”, it would seem natural to designate
the coloration as “concealing”. The high pilosity of the open
desert forms merits further study.

Another excellent recent example, the result of careful field
study of a fauna inhabiting a region with well-marked ecological
characteristics, is the work of Linsdale {1938) on the avifaima
of the Great Basin in the western U.S.A. The region is arid,
the climate severe, with prevalence of strong winds and some-
what scanty and usually low vegetation; the distribution of most
birds therefore tends to be more scattered than in more luxuriant
surroundings. The preponderating charaaers of the passerine
birds correlated with those environmental features are as follows:
a great development of flaght-songs, in relation to the scarcity
of high perches; a high percentage of protectively-coloured
adults; a tendency for both nestling plumage and nest-hning to
be pale-coloured, in order to reflect excessive Hght;* strong
powers of flight, to cope with the wind; a high proportion of
species are migratory, in relation to the severity oi the climate;
songs and calls are unusually loud, to compensate for the scattered
distribution of individuals; long-range vision is unusually acute,
partly for the same reason, partly in correlation with the lack
of obstruction by vegetation; a high proportion of forms nest
on or close to the ground.

Dice (1940/1) calls attention to adaptive regularities among the
subspecies of the single genus Peromyscus, and we mention else-

* Linsdale (1936) has also shown that the opposite conditions ate correlated
with dark nest-Uning and nestling plumage, thus facilitating die maximum

absorption of heat

ADAPTATION AND SELECTION 433

where (p. 214) the similar regularities in the Australian bird
Acanthiza.

We have already referred to the frequent correlation of general
tint with climate (Gloger’s rule, p. 273). Meinertzhagen (1934)
gives a good example of this, in the darker plumage of a number
of bird species in the Outer Hebrides. He concludes that reduced
sunshine and increased atmospheric humidity, rather than higher
rainfall, are the meteorological factors responsible.

Meinertzhagen (1919) also points out that in migratory species
these regularities are correlated only with th 6 climate of the
breeding-quarters, not at all with that of the winter-quarters.
This may be due to the greater intensity of selection during the
breeding-season (cf. p. 212).

Buxton (1923, ch. 7) gives a valuable summary of the colora-
tion of desert animals ; but liis rejection of their cr}’'ptic selective
value is much too sweeping. Though doubtless many instances
of sandy pallor in deserts are examples of Gloger's rule, and
correlated primarily with climate, many • others are certainly
cryptic. His objection that normally invisible areas, such as the
soles of the feet in mice, are of the same colour as the visible
parts- may be accounted for by “correlated variation”, the entire
colour being affected except where selective counter-reasons
exist. In general, pigmented chitin is tougher, more heat-
absorptive, and less permeable to water-vapour. This accounts
for various regularities of insect distribution (Kalmus, 1941b),
e.g. the frequency of black desert species (p. 451), and the increase
of pigmentation with altitude and latitude.

It should be mentioned here that some bird species have been
experimentally darkened by exposure to humid conditions. The
most interesting case for our purpose is Munia jlaviprymna, a
desert form of weaver from Australia (Setb-Smith, 1907).
The dark experimental modification of this form, though rather
variable, is somewhat similar to a dark form found in nature in
a more humid region of Australia. It was at first concluded that
die dark colour of this latter form (which was treated by Seth-
Smith as a distinct species, but is to-day regarded as subspecific)
was itself only modificational. It is much more hkely, however.

434 EVOLUTION : THE MODERN SYNTHESIS

especially in view of its greater variability, that the
darkened desert form was what Goldschmidt (e.g. I94 )
a phenocopy of the genetic darkoiing of the subspea^ from he
h4iid region. If so, the two contrasted forms may have arisen
bv organic selection (p. 304). genetic adaptation havmg replaced
aA original adaptive modification. In any case it is worth notmg
that other climatic colour-forms are not modifiable m this way.
Thus Sumner ( 1932 , P- ^6) could obtain no dartag of
pale forms of desert Peromysats m more hmrnd conditions, or
lightening of dark humid forms in drier conditions.

When ataptive regularities exist, any excepnons to them
immediately attract attention and call for andysis. For instance,
the correlation of some sort of webbing on the feet with markedly
aquatic habits is all but universal in birds. Ducb, geese, swans,
gk, terns, petrels, frigate birds, peUcans, cormormte, ganneK,
iid the like have either three or aU four toes jomed by a web;
coots, moorhens, grebes, and phalaropes have lateral lobes on
ea ch toe. But the dippers (Cinclus) exhibit not a trace of -webbrng
or any other aquatic adaptation, although they are restricted to
streams, obtain much of their food below the surface of the
water, and can swim on the surface. Structurally, they appear as
terrestrial as a thrush or a ivren. Can there be a re^on for this
exception to the rule, or are they stiU in the early stages of
adaptation to a new habitat ? Their wide distribution seems to
negative the latter explanation. The suggestion may be made

that they have adopted a unique type of aquatic food-seeking.
Many birds that frequent stream-edges walk some way mto the
water in order to find food; the dippers have extended this
habit and walk on until they are wholly submerged. They search
for food by subaqueous walking, and in this they not only do
not require webbing but can get a better grip of submerged
water-plants and rough surfaces if their toes arc free. The excep-
tion is a clue to exceptional habits.

Determination of metabohsm, temperature-resistance, etc.,
when combined with accurate anatomico-physiological study of
respiration and directed by ecological knowledge, often reveal

arlaDtation of forms to their habitat.

ADAPTATIGN AN0 SEtECTION

435

f As an example we may take the work of Wingfield (19393, b)
on mayfly nymphs belonging to various genera. Thus in Baetis
from swift streams, the tracheal gills do not aid oxygen-con-
sumption; in the pond-dwelling Chloeon dipterum they act as an
accessory respiratory mechanism by promoting ventilation, but
I at low oxygen concentrations only; while in the burrowing

I Ephemera vulgata they aid oxygen-consumption in all circum-

j stances, apparendy as true respiratory organs as well as by pro-

viding ventilation. Similarly, forms from swift streams have a
lower thermal tolerance than those from slow streams, while those
from ponds are most resistant. This is in accordance wfti the tem-
perature extremes expected in nature (see also Whimey, 1939).

Fox and his co-workers (see H. M. Fox, 1939) have studied
the activity and metaboHsm of poikilothermal animals of very
various kinds from different latitudes. Among closely-related
species, the one living in higher latitudes is generally, but by no
means always, adapted in some way to the lower temperatures
of its normal habitat: at a given temperature, its heart-beat,
rt'spiratory movements, or other activity, is greater than that of
its relative from warmer regions. The same phenomenon may
also be found as between high-latitude and low-latitude popula-
tions of die same species. Differential heat-resistance also exists
in many cases. As Fox points out, it is difficult to be sure whether
the undoubted adaptation thus shown, enabling cold-water types
to carry on the business of living at a reasonable rate, is modifi-
cationai, genetic, or a mixture of the two. Wc are probably safe
in assuming that, when die difference is one between different
species and is of considerable extent, it is mainly genetic, although
the recent work of Mellanby (1940) shows how rapid and
extensive modificational adaptation may be. A critical analysis
of the problem is highly desirable. (Cf. calcicolc plants, p. 273.)
J. A. Moore (1941) has demonstrated a similar and undoubtedly
genetic adaptation in different species of frogs, those from colder
breeding habitats having a lower tcmperaturc-tolcrancc, and
faster-developing eggs. Even the jelly-membranes and the form
of the egg-mass are climatically adapted (Moore, i94o)- Again,
the field-mouse Apodemus jiavkotUs, in correlation with its

436

evolution: the modern synthesis

distribution, prefers rather lower temperatures dian the closely
rehted A. syhaticus (Kalabuchov, 1939); furthermore, within
the species, individuals from higher latitudes preferred lower
temperatures than those from warmer regions. (See p. 271.)

That the adaptation between geographical varieties or sub-
species of’ a single species may also be mainly genetic is shown
by various researches, such as Timofeeff-Ressovsky’s previously
cited work on local variation of temperature-resistance in Droso-
phila funehris (p. 191), but most exhaustively by the studies of
Goldschm^t (1934, 193 8&) on the gipsy moth Lymantria dispar.
Here the genetic peculiarities of the geographic race, to use
Goldschmidt’s own words, “harmonize the life-cycle of the
anitnal, especially the feeding season and the diapause, with the
seasonal cycle of the inhabited region”.

In many cases, notably in Japan, the lines of genetic demar-
cation between major groups of races are quite sharp. Originally
it had been found impossible to correlate feese lines with corre-
sponding sharp changes in any single meteorological factor.
Recently, however, as Goldsch^dt (193 8l») points out, it has
been shown that they correspond -with extreme accuracy with
changes in soil type, and that the soil types in their turn depend
upon the interrelation of several meteorological factors. This is
a reminder of the fact that climate cannot be properly measured
by variations in single meteorological phenomena, such as tem-
perature or day-length, since it inevitably represents a complex
summation of numerous factors; and further, that physical
factois like soil or biological features such as geographical distri-
bution may often prove the best indicators of such summations.
In this instance, the discovery of physiological adaptations between
subspecies of moths proved to be the first (and very accurate)
indication of climatic regional difierences. It should, however,
be noted that though the subspeciation of the gipsy moth is
thus delicately ac^usted to climate, adaptation to food plants
may art as a Hmiting factor (see later for cases of climatic limiting
adaptations). For instance, in the U.S.S.R., the area of periodic
mass outbreaks of rapid reproduction of L. dispar coincides with
the distribution of its optimal food, the oak plant.

437

ADAPTATION AND SELECTION

p lan ts; too, may show delicate climatic adaptation of geo-
graphical (ccochmatic) subspecies. Thus according to Clausen,
Keck, and Hiesey (1937) the coastal subspecies (wliich they call
ecotypes) of most Cahfornian plants have a constitution genetically
harmonized with a chmate providing mild winters and along
growing period. Transplanted to an alpine station, although
development is hastened through dwarfing, they can
seldom or never mature any seed, and are often unable to flower
on resuming growth in the summer. The alpine subsides
(“ecotypes”) of the same plant have a cycle related to a chmatc
of long cold winters and a short growmg period. Transplanted
to a coastal station, they flower poorly or not at all, and show
a generally weak appearance in spring. “The adaptive capacity
(modificational plasticity) of coastal and alpme ecotypes is there-
fore insufficient to allow either to live and to compete m the
habitat of the other. It is the diflference in inheritance that enables
them to succeed in their respective regions.”

The exhaustive experimental studies of Turesson (see sum-
mary in Barton-Wright, 1932) have independently led km to
similar conclusions. In different regions, adaptations arise wkch
are jointly related to cHmate and Hfe-cycle. He investigated both
summer-flowering (aestival) and spring-flowermg types, i
aestival forms, the more southerly populations showed a con-
siderable (genetic) increase in height combmed vnth lateness,
wkle alpine populations showed earliness and reduced height
as compared with lowland ones. In spring-flowering forms, on
the other hand, it is the more northerly pop^ttom that show
lateness, up to the latitude of southern Sweden; further north
than tks, earliness is again favoured. The low-latitude earhness
appears to be related to the general earliness of trees in t e
region, for it is advant^eous for the spring herbs to produce
their leaves and flowers before the leafy canopy cuts 0
sunlkht; in very high-latitude spring forms, earliness is doubt-
less correlated with the shortness of the vegetation penod. All the
regional peGuliarities of the plants investigated are t us a

Similar though less exact conclusions are reachc Y ■ '

White (1926). For instance, black wahiuts (Juglans mgra) from

438 EVOIUTION; THE MODERN SYNTHESIS

Miinesota are muct winter-hardier than those from Bahama
or Texas, though morphologically indistinguishable. Ag^, a
bigb mountain ecotype of the Cedar of Lebmon ( e rus t}am)
is perfectly hardy in Massachusetts, where the normal term ot
this species shows poor cold-resistance. In support of adaptive
climatic diBferentiation within the species. White cites toe com-
mon practice of gardeners and foresters to use seed from^ the
northern limit of a species’ range when winter-hardiness is desired.

4. ADAPTATION AS A RELATIVE CONCEPT

In cases like these, the physiological characters of the local groups
must clearly have been adjusted during evolution to the climatic
characters of their environments, and are thus in the strictest
sense adaptive. But there are many examples where the evolu-
tionary relation between physiology and chmate is not so obvious.
As illustration we may take some of the cases of plant distri-
bution in Britain so interestingly discussed by Salisbury (i939)-
In the Scots pine {Pinus sylvestris), pollination occurs normally
in May, but fertilization not for another thirteen montlis. Unless
the temperature in both summers reaches a certain minimum
combination the pollen-tubes will not reach the ovules. This
provides quite a different set of meteorological conditions for
fulfilment than does the attainment of a minimum level of
temperature during one season, as would be the case for the
fertihzation of most species, and there is some evidence to show
that it is a limiting factor for the northern distribution of the
species.

In many cases it is the temperature obtaining during the time
of fruit-formation, not flower-formation, which is decisive. This
is so, for instance, with many species of the southern element
in the British flora, such as the common milkwort,
calcarea, or the fluellin, Limrk spuria. The form of the life-history
may be of importance in various ways. For instance, the time at
which flower-buds are laid down varies in different plants. In
daffodils it is May, and the optimum is about 9°C., while in
hyacinths it is August and the optimum about 25°. Thus in

ADAPTATION AND SELECTION 439

daifodik either too high or too low May temperatures would
inhibit flower production, while they would have no influence
on the process in hyacinths. Again, the two British species of
Aruni differ in their winter habits, the common cuckoo-pint,
A. tmculatum, over-wintering as a deep-situated corm, immtine
from most frosts, while A. neglectum produces its new fohage in
December. The latter is thus readily lolled by frost, but where
it can survive, its winter photosynthesis gives it an early start in
spring before the trees above it have developed their leaves.
The British range of A. macuhtum extends far into Scotland,
whereas that of A. neglectum is restricted to our southern and
south-western coasts. We may say that A. neglectum shows an
adaptation to woodland life — ^but only in ntild temperate cUmates.
A somewhat similar diSerence, with similar results on distri-
bution, is seen between Scilla verna and S. autumnalis.

Numerous similar cases may be found in textbooks of plant
ecology. We may add a recent example from animak. Nash
(1937) has been able to study the ecology of the tsetse-fly Glossina
morsitans both in East and West Africa (Tanganyika and Nortliem
Nigeria).* Both races appear to demand the same or very similar
optimum conditions — ^a temperature of about 23“ C. and a
saturation deficiency of about 6 milHbars. These conditions are
much more nearly reproduced in the Berlinia-Brachystegia wood-
lands of Tanganyika than in the rather different habitat provided
by the small residual forest islands of North Nigeria. In both
regions these forest areas constitute the “true habitat” of the
species. In the dry season, as evaporation rises the flies become
restricted to this true habitat; but in the wet season they show
a much wider dispersal. Distribution is definitely controlled by
climatic factors, not by abundance of game for feeding.

In Tanganyika, the species breeds mainly under fallen trees;
in the rainy season dispersal is very extensive, and the comparative
mildness of the dry seasons may allow it to consohdate some of
its wet season advances and to form new fly-belts. In North
Nigeria, on the other hand, the species breeds promiscuously on

* The West African form is often distinguished as a separate species, G.
suhmcrsitans; but is better regarded as a geographical subspecies.

440 evolution: THE MODERN SYNTHESIS

the forest floor (so that the logtraps so valuable in East Africa are
useless); the wet season dispersal is much less in extent, and the
severity of the dry season is such that no new colonization can
occur; the concentration of fly during the dry season is much
more pronounced; the heat and aridity of the dry season is so
great that certain habitats (small meadow-pans) are never available,
and mid-day inactivity (never observed in East Africa) occurs.

Nash considers that the West African form has remained essen-
tially similar to the East African in its physiological requirements.
“Having failed to adapt its constitution to the climate, it has
perforce adapted its habits; had it evolved a constitution which
preferred a higher degree of evaporation and temperature, the
greater frequency of optimum conditions would have enabled
it to become as widespread a pest as its East African represen-
tative.”

In a later paper Nash (1940) has applied these theoretical
considerations in practice. Dealing with the three species of
Glossim found in Nigeria, G. tachinoides, G. palpalis, and G.
moTsitans, he first established their basic ecological relations, and
then introduced experimental clearing designed to accentuate
the severity of pessimum conditions. With G. tachinoides, partial
clearing on a small scale leads to local extermination, but this is
followed by recolonization. With large-scale clearing, however,
total extermination is obtained. With G. palpalis, this method
appears to be of value only in the drier parts of the species’
range. Finally, with G. morsitans, which has rather diferent
ecological requirements, very extensive and ruthless total clearing
is needed to eflect extermination, and is not recommended
“unless warranted by a large [human] population and abundant
funds”. The case is interesting as illustrating the practical applic-
ability of an ecological viewpoint which thinks in terms of
adaptation to environment.

These examples from tsetse-flies are illuminating in various
ways. They illustrate, hke the plant examples previously adduced,
the importance of inherent physiological requirements, but also
well demonstrate the role
adaptation.

of modificational plasticity in ensuring

ADAPTATION AND SELECTION

As would be expected, plasticity in this sense is more wide-
spread among plants than among animals. Following up the
pioneer work ol Bonnier (1895), its extent has been investigated
by various authors. Many forms have an astonisliing degree of
plasticity. Thus Clements (1929) was able to demonstrate marked
alpine dwarfing in lowland types of many species transplanted
into alpine conditions. He was at first inclined to minimize the
existence of genetic differences between types. Later, however,
(c.g. Clements, 1933) he admits that species differ in their plas-
sticity. Thus in the genus Mertensia, M. sibirica has no plasticity,
while M. pratensis and M. lanceolata can be made to resemble
each other very closely.

Clausen and his associates (see e.g. Clausen, Keck, and Hicscy,
1938, 1940) have shown how complex is the interrelation of
genetic and modificational factors in such cases. For instance,
four major ecotypes (ecoclimatic subspecies) are differentiated
in the majority of plants in the U.S. Pacific slope— a coast range,
a lower mountain, a subalpine, and an alpine form. Yet corre-
sponding ecotypes of different species may react quite differently
when transplanted. Thus the alpine race of Potentilla diversifolia
is relatively stunted when transplanted to a mid-altitude station
(though near sea-level it again becomes larger) ; but the alpine
races of P. glanMosa and P. gracilis become largest at the mid-
altitude station and are most dwarfed in their natural habitat:
the alpine races of Achillea millefolium and Aster occidentalis, on
the other hand, while tallest at tlie mid-altitude station, arc more
dwarfed in lowland than in alpine conditions.

Meanwliile Marsdcn-Joncs and Turrill (1938, etc.), though
failing to corroborate some of the more sweeping claims of
Bonnier and of Clements (see discussion in Turrill, 1940), have
demonstrated how diftcrent is the range of modificational plas-
ticity in different species. Thus the knapweed Centaurea nemoralis
and the kidney-vetch Anthyllis mlneraria arc little modifiable
by different soil conditions, while the plantain Plantago major
is extremely plastic. In higher animals, behavioural adaptation
seems to take the place of modificational plasticity in plants.

In some of these cases, the modification can hardly be regarded

442 evolution: THE MODERN SYNTHESIS

as adaptive. This applies, for instance, to die stunting of organisms
by unfavourable fonditions as the limit of the range of the
species is approached. This is of course common in plants, but
may also occur in higher animals. Thus the American freshwater
fish Xenotis megalotis is markedly smaller in the northern part
of its range, in correlation with the mean temperature and the
length of the warm season (Hubbs and Cooper, I 935 )- Fully-
grown forms from northern Michigan are 20 per cent smaller
than those from the south of the state. Though such modification
appears to be wholly non-genetic, it must alter the ecological
relations of the species.

These examples of correlation between organic constitution
and climate or habitat begin to shed light on the problem of
adaptation as a whole. Some climatic adaptations show hgh
specialization — ^for instance, the run-off mechanisms of plants
exposed to constant moisture, or contrariwise the water-storage
mechanisms of certain desert plants; some of these latter from
the Arizona desert can store enoigh water to last for more than
one rainless year — ^in certain cases (e.g. Ibervillea somrae) up to
ten or more! (see MacDougal, 1912). Other correlations with
climate are more general, though clearly adaptive in the narrow
sense of having been accumulated by selection over a long period.
Here we may reckon the various adaptations of plants to cold
winters— deciduousness in broad-leaved trees; restriction of
transpiration in needle-leaved trees; over-wintering as bulbs,
conns, or seeds, etc., in herbs; general resistance to low tempera-
ture. Adaptations of mammals in cold climates to hibernation,
to the reduction of heat-loss, or to adjust the breeding season to
the needs of the growing young, fall under die same head. For
example, in the roedeer fertilization occurs in autimm as with
other north temperate Cervidae, but the embryo does not
develop beyond the early segmentation stage until spring, thus
ensuring birth in the favourable period of early stunmer (see
P. H. A. Marshall, 1910, p. 32). Similar definite climatic adap-
tations, but of a much more delicate nature, are to be found
between closely related species, or, as we have seen in Lymemtria,
between races of a single species.

' ADAPTATION AH 0 SELECTION .. 44 .^

At the Other extreme there arc organisms with ranges liiiiited
by elimatic factors, rather than closely adjusted to them. We
have just seen excellent examples in Glossina morshmts and in
various plants. Such forms of course show some climatic adap-
tation — ^no tsetse-fly could exist in the arctic, for instance, or in
a full desert — ^but it is of a very general nature. The correlation
between the organism and its environment is in tliis respect
neither delicate nor exact: dicre is an absence of the lock-and-kcy
correspondence to be seen, for instance, in some colour-adap-
tations, or in various devices for securing cross-pollination in
plants — ^and apparently in the climatic relations of Lymantrk.
Similarly, many higher animals are found in a number of different
habitats. Adaptation is then to a range of habitat-types, not to
a single habitat. Certain features in the environment (here often
in the plants rather than in physical characters) act 'as limits to
the distribution of the species, but adaptation is not close or
detailed (see Diver, 1938, 1940).

The common heron {Ardca cinerea) shows a marked ecobiotic
adaptation to securing food from shallow waters. In addition,
it is restricted, and presumably adapted,, to a certain climatic
range. But the environment also acts selectively in yet another
way. During exceptionally severe winters, herons may starve
through the freezing of the waters wliich they frequent. The
careful records compiled annually by the British Trust for
Ornithology (Alexander, 1941) show that herons from colonies
within easy range of salt .water were least affected by the very
severe winter of 1939-40. In 1940, the heron population of the
British Isles showed a general decrease in number of occupied
nests, compared with the average for the previous three years.
But whereas for inland heronries (more than 25 miles from
tidal waters) the decrease was 31 per cent and for those between
2 and 25 miles from tidal waters it was 26 per cent, for coastal
heronries (less than 2 miles from tidal waters) it was only 13 per
cent. Thus low winter temperatures and distance from the sea,
sometimes separately, sometimes jointly, are bound to be factors
limiting the northern distribution of the species. This is borne
out by the facts. The heron breeds up to 70^ N. in Norway

444^^^ MODERN SYNTHESIS

but only to 66“ in Sweden and to about 6 o° in the U.S.S.R.
( Witherby, 1938-41).

We find fiirther that organisms may be adapted to climatic
(and other environmental) factors either narrowly or broadly.
Stenothennic species, for instance, have a narrow range of
temperature-tolerance, eurythermic forms a wide one; Moore
(1940, Amer. Nat. 74 ; 188) points out that eurythermy is very
rare in aquatic animals, and is itself an adaptation to the fluctuating
temperature of land life. Stcnohalme and euryhaline forms may
be similarly distinguished. We may extend the concept to indi-
vidual plasticity by distinguishing “stenoplastic” and “eury-
plastic” forms (p. 519). Euryplasty may grade over into general
high viability, which is itself an adaptation, though internal or
intrinsic rather than related to particular external conditions.
Range of ecological habitat may sdso be broad or narrow.

We must also remember that adaptations may be very close
and detailed, and yet, like mimicry in Lepidoptera, of no or
negligible value to the species as a whole, since they have arisen
entirely by intra-specific selection (§ 8), and are thus biologically
subsidiary to adaptations affecting general viabihtjr, resistance to
parasitoids, etc. (see A. J. Nicholson, 1927).

This is perhaps the place to mention some interesting cases
which suggest that evolutionary adaptation to recent cUmatic
change may now be active. I refer to the numerous well-authen-
ticated cases of steady and considerable extension of range which
cannot apparendy be put down directly or indireedy to human
interference. Timofeeff-Ressovsky (1940) cites several cases,
of which we may mention the serin finch (Sermus canarius serims)
and the yellow-breasted bunting [Emberiza aureola). The former
has since 1800 extended its range nordiwiurds from southern
France almost to the English Channel, and from the eastern Alps
almost to the Baltic, The latter since 1825 has extended its range
westwards from the Urals to west of Leniugrad, Similarly the
roller (Coracias garrulus) has shoym a northern range-extension
in eastern Europe, and the warbler Acanthopneuste viridana a
westward extension very similar to that of Emberiza aureola;
while the black redstart {Phoenicurus ochrurus) has more or less

445

ADAPTATION AND SELECTION

paralleled the serin, and the greater spotted woodpecker {Dryobates
major) has within fifty years extended its British range from
south of the Tweed to the northernmost woodlands in southern
Caithness (Witherby and others, 1938-41,}. Fisher, I940cj. The
last two cases are not cited by Timofeeff-Ressovsky.

Meinertzhagen (1919) cites other cases, such as the shore-
lark, Eremophila jiava, which has not only expanded its breeding-
range westwards, but about 1847 established a new migration
route, in this difiering from other species which have extended
their range in a similar way. He also mentions the crested lark,
Galerida cristata, as an example of the same phenomenon on a
more extended time-scale, and accompanied by subspeciation.
The fulmar petrel {Fulmarus g. glacialis) has shown a marked
southern extension of range within the last sixty years along tie
coasts of Britain. The old supposition that the spread was initiated
by a reduction of human depredations seems to be erroneous
(J. Fisher, 19400, b; Fisher and Waterston, I 94 i)- , , .

Among Lepidoptera, the moth Plusia nwneta, first recorded m
S. England in 1890, is now common there, and has reached
Scotland (South, 1939)- The comma butterfly, Potygonia c-albmi,

hardly known in Britain outside Gloucestershire, Monmouth-
shire, and Herefordshire before 1920, has since markedly extended
its range E., S., and N. (numerous reports in The Entomologist).

It may be suggested that, whenever the effects of human
interference can be shown not to have been operative, such
range-changes will generally be the result of a changed ecologica
adaptation. The ranges of forms hke the fulmar would be much
restricted by the amcUoration of climate since the last ice-age,
and any genetic changes in temperature-tolerance or n«t-sitc
selection which enabled the species to regain some of the lost
ground would be subject to positive selection. The other species
mentioned arc extending into milder climates; here presumably
a climatic preadaptation was already present, which changes m
habitat-preference or nest-site selection have finally cnab cc t le
species to utilize. The matter is a complex one, however, and
needs thorough investigation before we can conclude that the
range-changes are the result of adaptive change.

446 evolution: the modern synthesis

It must not be forgotten that, in the long perspective, d^amic
evolutionary trends are as important as are static interrelations
at any given moment. The worsening of the climate at the end
of the Mesozoic reduced the general adaptiveness of the dmosaurs,
pterosaurs, and other reptilian groups, while increasing ^^hat o^
the early mammals and birds. The recent glacial period enabled
the cold-climate preadaptation of many tetraploid plants to
become dominant over the other adaptive features of the corre-
sponding diploids in higher latitudes, leading to extensive spread
of the former. The spread of man favoured that of organisms
preadapted to be commensal or semi-parasitic on him or his crops,
like house-sparrow, rat, house-martin, or weeds in general.

Adaptation is thus seen, not as a hard-and-fast category, but
as something relative. It is not an all-or-nothing phenomenon,
but takes many forms and exists in all degrees. Like other
logical categories, it can only be properly understood by detailed
and where possible quantitative analysis. Furthermore, the
mistake must never be made of thinking of adaptational adjust-
ment solely or primarily in relation to the physical environment:
the biological environment is Just as important. In some cases
plants are restricted to special habitats not because of special
climatic adaptations but because they possess a wide range of toler-
ance towards climatic conditions, with a low degree of what we
may call biological or competitive vigour. Thus competition pre-
vents their establishment in most habitats; only where their extra
margin of tolerance removes them from the swamping effect
of their biological competitors can they flourish. Salisbury {1929)
cites various cases of this phenomenon. Thus RMunculus parvi-
Jlorus is in Britain restricted to very unfavourable habitats, e.g.
dry shallow soil overlying rock. In cultivated ground (unmanured)
it not only grows well, but much better than in nature, and
produces ten to twenty times as many fruits. Again, the sorrel
Rumex acetosa is notorious as a plant of acid soils. In cultivation,
however, it shows an increased growth on Hmed soils, proving
that its restricted distribution in nature is due to the competition
of plants which arc less tolerant of acid-conditions.

Such examples “sufficiently illustrate the fact that plants grow

: ADAFTATION AND . SELECTION - 447

not where they would, but rather where they must”. The same
sort of tiling is probably true of many plants characteristic of
marginal conditions, c.g. alpine habitats (see p. 274).

In a letter (5. iv. 1940) Professor Salisbury has kindly furnished
me with some further striking examples. One concerns the
rosette plants of open grazing land. For instance, Senecio campestris
and Filipendtda hexapetala ‘‘are confined, as wild plants, to our
chalk pastures, but their vigorous growth in other types of soil,
when they arc protected from competition by cultivation,
indicates that their restriction in nature is due to the competition
factor”. The continual grazing prevents other plants from
growing Iiigh enough to affect the rosette plants" growth, whereas
their peculiar growth-habit flattens their own leaves down in
such a way that they cannot readily be eaten.

Another example, involving quite different factors, is that of
the hellebore {Helleborus foetidt 4 s)y which in Britain is almost
entirely confined to ash woods on calcareous soils. Here again
in cultivation tliis species grows and reproduces well in non-
woodland and n on-calcareous situations. Its peculiar restriction
appears to be due to a combination of two factors. In the first
place, it seems susceptible to competition, and any woodland
habitat suppresses competitors which arc vegetatively active only
in summer, whereas it, being evergreen, can assimilate also
in winter. On the other hand, most woodlands are too shady
in summer; but the unshaded phase of ashwoods, wliich lasts
for seven months out of the twelve, is sufficient for the hellebore.
As Salisbury (1929) well says, “dominance may be the conse-
quence of unfavourable conditions actmg by selective depressiotiy
or to favourable conditions acting as a selective stimuhsy but in
cither case the dominance is determined by the relative vigour
of the species and its competitors”.

The perfection of adaptation is also correlated with the degree
of competition and other forms of selection-pressure. We discuss
this phenomenon more at length elsewhere (pp. 426, 469 seq.) ot
this chapter. Here we will merely recall the well-known fact
that the intensity of life in the tropics is correlated with a greater
prevalence and a greater perfection of various adaptations, ot

evolution: THE MODERN SYNTHESIS

nimicry is perhaps the best studied. Simdarly arbored
ons sui as prehensile tails are most fully developed
he arboreal habitat is developed in most extreme fashion
)Uth America. Conversely, where selection-pressure is
adaptation tends to be less perfect. We have seen a smal^-
ample of tWs in the cichlid fish of Afirican lakes (p. 324).
«cde example is provided by the marsupials of the
tan region. Tree-kangaroos, for instance, show an adap-
■o arboreal life so incomplete that one cannot miagme
[rvival in the tropical forests of Malaya or the Amazon.

general, the Australian marsupials seem unable to com-
iccessfully with introduced forms from other regions,
T predators or direct competitors.

brings us back to what has already been said about ada^
and function (p. 417). Adaptation, we there said, is
the problem of efficient fimction seen from a shghtiy
It angle But it is a commonplace that all grades oi
:cy of every function coexist in nature. The ftmction of
ranges from mere response to high light-intensities up to
liar colour-vision. Aquatic locomotion is at a low level
oeha^ at a high pitch in a dolphin or mackerel. Thus we
1 nature, not merely every possible type of adaptation,
^ery grade within each type. Efficiency of function at its
general consists in all-round viability, and this is largely a

of oarts and part-functions.

AN 0 SELECTION ■ ■ 449 '

young slielis of the same size, though the mean was the same.
Tins appears to indicate selective ehmination of extreme types.^

This is what we would expect on a selectionist view. Organ-
isms are selected, not on the basis of conformity to an ideal
plan, not in relation to complete functional ciEFicicncy, but on tlic
basis of survival. The forms that exist are tliose that have
managed to survive; and survival may be and often is achieved
by means of cur.ously makeshift devices. Not only that, but a
high degree of adaptation in one character or function may be
a measure of low efficiency in some other respect. It seems, for
instance, to be no chance that the most elaborate devices for
cross-pollination occur in somewhat rare species of orchids;
and Batesian mimicry can only develop in types which are much
rarer than their models. Again, specialization which brings
success in one set of conditions may involve a loss of plasticity,
and so be a real disadvantage if conditions change (see p. 377).

Thus the study of adaptation seems destined to take a new
turn. The first stage concerned itself with the fact of adaptation
— is such-and-such character an adaptation, or is it not? In the
next stage biologists were interested in the mechanism of adap-
tation — do adaptations arise through natural selection, by
Lamarckian means, or in what other way? To-day the emphasis
is on the analysis of adaptation itself, and the bearing of that
analysis on other branches of biology — ^how well-dcvclopcd arc
the different types of adaptations shown by a particular organism,
and what light docs its particular adaptive complex throw on
its ecology and on the direction and the strength of the selection
to which it has been exposed? The significance of adaptation can
i)nly be understood in relation to the total biology of the species,

5. PREADAPTATION

The subject of preadaptation demands a section to itself. By
prcadaptatioii (sometimes styled passive adaptation) wc mean

^ The criticisms of Robson and Richards (1936, p. 21 1) do not appear to be
peftinent. If, as they suggest, the young shells are more plastic, this should have
been revealed iii the inner whorls of old shells also. It is also difficult to see why
environmental agencies should always reduce adult variability as compared
wkhjuvcmle.

450 evolution: the modern synthesis

ffifKer diat an existing species (or subspecies) is by its peculiarities
predisposed to take advant^e of a certain type of environment,
or that a particular mutant or natural variety is from the outset
adapted to particular conditions, whether those in which it
originates, or others into which it might be thrown by chance.
We may distinguish the two as constitutional and mutational
preadaptation respectively. Let us take this latter category first.
Lamoreux and Hurt (1939) find that White Leghorn fowls are
markedly more resistant to vitamin B deficiency than other breeds,
such as Rhode Island Reds or, still more. Barred Plymouth
Rocks. On normal diet, this characteristic is without any effect
on survival, but on a somewhat deficient diet it could be decisive.

A somewhat similar type of variation in a physiological (and
therefore potentially adaptive) character is seen in the response
of the aop-sac of pigeons to the pituitary hormone known as
prokctin (R. W. Bates, Riddle, and Lahr, 1939). Some breeds
proved no less than eight times as responsive as others. Similarly,
among plants different strains of the same species may difier
markedly, e.g. in water-requirements.

We have already drawn attention (p. 118) to the marked pre-
adaptation of certain mutants in fowls to warm climates, a
preadaptation which has been taken advantage of by man. Hutt
{1938) has shown that other breeds show minor differences in
genetic heat-resisting capacity, which could well serve as pre-
adaptive features.

An interesting case was found by Strohl and Kohler (1934)
in the meal-moth Ephestk kuhniella. Here a mutation to brown
colour, thoi^h accompanied by certain unfavourable properties
— ^reduction in egg-number and length of life— -also involved a
markedly higher heat-tolerance. This differs from the thermal
preadaptation of the cladoceran mutant previously described
(p. 52) in the complex of characters involved, one of them a
visible colour-mutation. What appears to be an example of
mutational preadaptation is the replacement of normal by
melanic forms in various warm-blooded vertebrates in certain
areas (p. 104). As pointed out, the dark forms appear to be
preadapted to a moister and cooler climate.

apaptation and selection 451

Polyploidy in plants is frequently a thermal preadaptation, but
in this case usually towards cold-resistance (p. 337)- An excellent
example of preadaptation in a hybrid is the rice-grass Spartina
tommndii, which has proved better adapted than either of its
parent species to their own habitat of saline marsh and mud-flats
(p. 341). In general, it is clear that any form arising by a sudden
large change, as by autopolyploidy, or still more by hybridiza.-
tion and allopolyploidy, must be preadapted in some way if it

is to survive (p. 349)- ^

Another possible case of preadaptation, here as regards colora-
tion, is that of the lapwing Lobipluvia malabarka, which on a
belt of brick-red soil along the Malabar coast lays highly cryptic
red eggs in place of the “earthy-coloured” ones seen elsewhere
/Baker I 03 i). As suggested in Nature (February 13. 1932, p- 247),
Sfmay be L to local selection of types laying the erythrystic
eggs found sporadically in so many species. However, the farts
concerning egg-mimicry in cuckoos cannot be explained on the
basis of preadaptation, and show that elaborate true adaptations
may be brought about in egg-colour, so that further analysis
of this case is required.

Kalmus (1941a and i) finds that various body-colour mutations
in Drosophila arc potential preadaptations to changes in humidity.
Thus yellow flics are less resistant to desiccation than wild-type,
but ebony and black flies are more so. This appears to depend on
the fact that darkening of the cuticle is associated with a tanning
process which renders it less pervious (p. 433; Pryor, 1940), a fact
probably to be connected with the frequency of black msccts

It is of course true that many such preadapted and markedly
distinct new forms are later modified by the selection of small
gene-mutations; and it is equally obvious that even the most
triflingly beneficial gene-mutation to be found m the consti-
tution of a wild species, must in one sense luve been preadapted
at its first occurrence. But there is a real and important distinction
between the two types of occurrence. For one thing, inany
(or most) gene-mutations appear to be of necessity carried on
in the recessive state until such time as they can be made part

452 evolution: the modern synthesis

of some especially favourable combmation: when this is so. Aey
were not preadapted as regards their original phenotypic effect.
The truth, as so often in biology, is that a continuous senes
but that the two ends of it are ve^ distinct. In generd,
when the or igin of a successful new form is due solely or ma^y
to a single large step (or at least to one that is readily perceived
as large by the biologist) we speak of mutational preadaptation
but when a new form arises by a series of small and in gener^
imperceptible stages, we speak of adaptation in the accepted
Darwinian sense.

We next come to constitutional preadaptation, where the
pyisring constitution of a stock predisposes it to certain modes
of life rather than to others. Salisbury (1929) points out that
annual plant species are preadapted to desert conditions. Another
example is afforded by the adhesive digital pads or discs of
various frogs, which are best developed in the arboreal tree-
frogs, though also present in My functional form in various
non-arboreal types (Noble and Jaeckle, 1928). Adaptation to
tree-Hfe here seems to have been secured by enlarging these pre-
existing structures relatively to body-size.* Engels (1940) shows
that the digging habit of ^e thrashers (Toxostotna) depends on
pre-adaptive peculiarities of musculature — an interesting case of
structure preceding ftuiction in birds.

In other cases, the preadaptation is less immediate, an organ
subserving one function being readily modified for another.
The classical example is the evolution of the lungs of land verte-
brates from the air-bladders of certMn fish, but there are of
course numerous other cases of Fmktionswechsel which illustrate
this long-grange type of constitutional preadaptation. In all these,
however, a great deal of adaptation in the ordinary sense is also
necessary, so that it could be better to exclude them from the
category of preadaptation proper, and style them predisposition.

General predisposition is shown in the ease with which second-
ary aquatic life is resumed by terrestrial types. Terrestrial Ufe

* It is fair to state that some authors would not exclude the hypothesis that
the non-arhoreal disc-possessing forms are secondarily derived from arboreal
forms.

ADAPTATION AND SELECTION 453

involves numerous progressive advances (Chap. lo, pp. 563-4) in
general physiology; the possession of these predisposes such
forms to be able to compete successfully with aquatic types m
their own environment. Predisposition in the endocrine field is
found where an organism which lacks a certain hormone, yet
contains tissues capable of responding to that hormone. An
example is the response of bird oviducts to progesterone, which
appears to be produced only by mammals (Riddle, Bates, and
others, 1938).

Returning to true constitutional preadaptation, we have tlie
well-wom example of flightlessness in the insects of small
oceanic islands. Although very numerous groups may be repre-
sented among them, a disproportionate number belong to groups
which are not m general good fliers, or are characterized by
reduced wings. Flightlessness is here thus the accentuation of a
pre-existing tendency. We may here mention the interesting
experimental results of L*Heritier, Neefs, and Teissier (1937) on
vestigial, the wingless mutant of Drosophila. When a mixed
population of winged (wild type) and functionally wingless
(vestigial) individuals was reared in the open air in such a way
that they were moderately exposed to the wind, the result after
thirty-eight days was an increase in the percentage of homozy-
gous vestigials from 12-5 to 67 per cent, through the wind
carrying away more of the winged flies. When the culture was
transferred to a large room, the wind could no longer act as a
selective agent, and in fifteen further days the percentage of
pure vestigials was halved.

A less familiar example is cited by Eigenmami (1909), who
maintains that modem fresh-water fish must have been recruited
from ancestors preadapted to fresh-water existence by possessing
non-pelagic types of eggs. But the locus classicus of discussion
concerning preadaptation is the blind cave fauna. The out-and-
out Darwinians believe that their sightlessness is due to selection
gradually ridding the stock of useless organs, while some out-
androut preadaptionists have gone so far as to maintain that
mutational blindness came first, and that the sightless type then
found a favourable environment ready-made in caves. The

454 evolution; THE MODERN SYNTHESIS

triith' would appear to be between these two views. No proof
has ever been given of full mutational preadaptation in this
case, and it is in any event most unlikely. But it is a fact that
die cave fauna is drawn preponderandy from types that normally
shun light and therefore live in holes and comers. Such forms
are constitutionally preadapted to enter caves, and will fre-
quently be visually under-equipped. Their later evolution ■mil
consist in their further adaptation to a completely cavemicolous
existence, accompanied by further reduction of eyes.

Thus Ei^nmann {1909) points out that the fish fauna of the
Kentucky caves must have been sifted out by this type of pre-
adaptation from a normal riverine fish fauna when a certain
stretch of river became subterranean. The types that were
negatively hehotropic, nocturnal, or stereotropic remained sub-
terranean, and then developed further adaptations to cave life;
while other ecological types moved out into a coimecting river
which remained in the open. “The major adaptation to cave
existence, the power of fcding their food and mates without
the use of light, they [the ancestors of the existing cave-fish]
possessed before the formation of the caves, and it is responsible
for their present habitat.” The same general ■view, with certain
modifications, is taken by more recent workers in this field.
Hubbs (1938), for instance, after presenting an analysis of the
characters and relationship of the thirty-five or so known cave-
fish, concludes that this “confirms the theory that cave animals
have arisen from species moderately preadapted to cave life”.
To take an example, the “weak-eyed, long-barbelled, nocturnal
catfishes” have given rise to an undue percentage of cave-fish.

There is one notabk exception to the general rule, namely
the Mexican <^zradd civc-Bsh, Anoptichthys jordani. Although
this, as its name implies, is blind, it must have been derived
from a form very similar to Astyanax fasciatus, which is a large-
eyed open-water form, -without any obvious preadaptation to
caves. Hubbs suggests that the lack of competition, as evidenced
by the absence of other cave-fish in this region, facilitated its
colonization of caves, sightlessness evolving later.

Hubbs concludes by pointing out that preadaptation has con-

ADAPTATION AND SELECTION 455

stituted but the first step in the evolution of cavc-fishes: later
changes, such as further degeneration of skin pigmentation and
eyes, and further specialization of sensory barbels and the like,
must have been produced by progressive adaptation after the
cave habit had been established.

It may be pointed out that, in general, the preponderance of
degenerative (|oss) mutation will result in degeneration of an
organ when it becomes useless and selection is accordingly no
longer acting on it to keep it up to the mark (p. 476 ; Muller, 1939).
In other cases, as in the hind Umbs of Cetacea, degeneration
may be actively promoted by selection, as the organ’s presence
externally is disadvantageous (for vestiges, see Huxley, 1932).

Thus, while normal Darwinian adaptation adjusts a species
to a constant or a changing environment in situ, constitutional
preadaptation acts as a preliminary sifting device, restricting the
inhabitants of specialized habitats in the main to forms with
some definite predisposition to the pccuHar mode of life involved.
Mutational preadaptation is intermediate in the nature of its
action, providing a prehminary sifting of lesser extent and
shorter range.

Some writers, e.g. O. E. White (1926), consider that a con-
stitutional preadaptation towards cold-resistance has led to
certain natural orders of plants being able to survive in higher
latitudes when the uniform warm conditions of the earlier
cenozoic later give place to a sharply-zoned chmate, while other
groups, not similarly preadapted, became restricted to the tropics.
Among die former, he cites the willows (Salicaceae) and horse-
tails (Equisetaceae), among the latter the palms (Palmaceae) and
the Artocarpaceae.

A somewhat different constitutional preadaptation to tempera-
ture is found in Crustacea (Panikkair, 1940). The osmoregulatory
mechanism of various marine Crustacea is such that they are
able to tolerate waters of low salinity much more readily at high
than at low temperatures. This fact is reflected in the natural
distribution of fresh- and brackish-water Crustacea, and very
possibly of other invertebrate groups.

Goldschmidt in various of his writings (see 1940, p. 390) has

456 evolution: thu modern synthesis

suggested that preadaptation may play a rather
means of large mutations giving what he "

sters” which then can serve as the startmg-pomt for quite ne>v
evolutionary trends. As one example of where he thinks this
must have occurred he cites the flatfishes, since he
impossible for their asymmetry to have arisen gradually. He
then extends the principle to other less copnt cases. Mr. J.
Norman, of the British Museum (Natural History), however,
tells me that there exist a few less extreme forms of flathsl ,
which must be similar to earlier evolutionary st^es, and that
there seems to be no reason against assuming a gradual evolution
of the group from the beginning. Many fish occasmnally rest on
the bottom, some on their bellies, others on then sides. If bentbe
existence for any reason were advantageous, selection would set
in to improve the type m this respect— with die belly-restmg
forms by dorso-ventral flattenmg and lateral extension has
happened in sharks and rays), with the sideways-resting forms
by behavioural and structural asymmetry of the eyes and head.

However, Goldschmidt goes further than diis. b bs latest
book { 1940 ) he mamtains that there is a fundamental distinction
between niiCTO- and macro-evolution. The former, depending
on gene mutation and recombmation, may lead to subspecihc
and other diversification within the species, but cannot pro uce
new species, or, 4 fortiori, higher categories. These come mto
being dirough macro-evolutionary change, wbch, accordmg to
him. demands a radical change b the primary cbomosomai
pattern or reaction-system. Such a change m reaction-system he
calls a systemic mutation, though he states that it may have to be
accomphshed m several steps. Only after the repattemmg has
reached a certam tbeshold value does the new species-typc
emerge. He considers ( 1940 , p- 207) that m some cases at least
the imtial stages arise only m the absence of selection-pre^ure
agamst the heterozygote and under certain conditions of in-
breeding. But once a new stable pattern, viable as a homozygote,
is produced, “selection acts only upon the new system as a
whole”, b other words, if it survives, it survives as a preadap-
tation m viabflity. b other cases he considers that the early

: ■ AD'APTaTI^ and SELECTION 457

steps, too, may be favoured by selection on account of viability
effects on development, and that the change will be oiiicli quicker
than any inicro-evolutionary effect dependent on single genes;
this process could be regarded as halfway between normal
adaptation and preadaptation.

1 do not propose to discuss these rather revolutionary views.
What has been said elsewhere shows that I disagree with them
in generaL There is a great deal of evidence that gene-mutations
arc involved in specific differences, and that subspecies may evolve
into full species. Many of Goldschmidt’s analogies between
‘mioiistroiis^’ forms found in nature and large mutational steps
observed in the laboratory (e.g. partial or total wing-rudinien-
tation) are valueless until we know that the natural forms have
arisen at a single bound; they may well be merely phcnotypicaliy
similar to the mutants, but be due to the accumulation of small
gene-mutations. Such accumulations may evolve into “gene-
patterns' ’ characteristic of species (Silow, 1941); and the asso-
ciation of gene-mutations with sectional rearrangements may
produce relatively large effects (p, 93): but these are not systemic
changes. When he states that the evolution of the Drepanididae
{see p. 325) “by a series of micromutarioiis controlled by selection
is simply unimaginable”, one can only reply that his imagina-
tion differs from that of many other biologists. He rightly
insists on the importance for evolution of mutations with conse-
quential developmental effects (p. 525); but these arc pre-
sumably gene-mutations (see also Waddington, 1941b).

However, even if we dismiss Goldschmidt’s views as unproven
or unnecessary, preadaptation of various kinds has clearly played
a not inconsiderable role in evolution.

6. THE ORIGIN OF ADAPTATIONS:. THE INADEQUACY
OF LAMARCKISM

How has adaptation been brought about? Modern science must
rule out special creation or divine guidance. It cannot well avoid
frowning upon cntclcchics and purposive vital urges. Bergson’s
elan vita! can serve as a symbolic description of the thrust of life

458 evolution: THE MODERN SYNTHESIS

during its evolution, but not as a scientific explanation. To read
L’Evolution Crhtice is to realiK that Bergson was a writer of
great vision but with little biological understanding, a good poet
but a bad scientist. To say that an adaptive trend towards a par-
ticular specialization or towards all-round biological efiGciency
is explained by an elan vital is like saying that the movement
of a railway train is “explained” by an elan locotnotif of the
engine. MoH^re poured ridicule on the similar pseudo-explana-
tions in vogue in the official medical thought of his day.

Modem biology, taken by and large, also repudiates lamarck-
ism. I need not refer to the lamarckian views of Bterary men
such as Samuel Butler and Bernard Shaw. They are based not
on scientific fact and method, but upon wish-fulfilment. Shaw,
in his preface to Back to Methuselah, says in effect that he dislikes
the idea of a blind mechanism such as Natural Selection under-
lying evolutionary change — ergo, such a blind mechanism cannot
{I had almost written “must not!”) be operative. Pace Mr. Shaw,
this reasoning does not commend itself to scientists. One of the
main achievements of science has been to reveal that the facts of
nature frequently fail to accord either with the wishes or witii
the apparcndy logical preconceptions of human beings. Per contra,
we may remind ourselves that, as was pointed out nearly half a
century ago by Ray Lankester (summarized by Poulton, 1937),
lamarckism is self-contradictory, since it maintains that “a past
of indefinite duration is powerless to control the present, while
the brief history of the present can readdy control the future”.

Nor need I go in detail through the wearisome discussion of
the various scientific “proofs” of lamarckian inheritance that
have been advanced. I would merely say that subsequent work
has either disproved Or failed to confirm the great majority of
them. An unfortunate suspicion rests on Kammerer’s work, and
his results on salamanders have not been confirmed by Herbst
(1924). Heslop Harrison’s adaptive induction of melanic muta-
tions in moths could not be re-obtained by McKenny Hughes
{1932) or by Thomsen and Lemcke (1933). Repetition of Guyer’s
work on induced inheritance of immunity by other investigators
has yielded entirely negative results (Huxley and Carr-Saunders,

ADAPTATION AND SELECTION 459

1924). Pavlov himself withdrew his claim to have demonstrated
the inheritance of experience in mice. Recently Crew (1936)
has repeated the elaborate researches of McDougall on the
hereditary transmission of the effects of training in rats: his
results entirely contradict McDougall’s lamarckian claims, and he
is inclined to ascribe the discrepancy to an insufficiency of controls
and an inadequate attention to genetic method on McDougal’s part.

Other work, such as that of Heslop Harrison on the feeding
habits of insects, is capable of alternative explanation, and is
therefore not crucial. Indeed, the researches of Thorpe (sec
p. 303) liave made the alternative explanation the more likely,
by demonstrating the role of larval conditioning to food in
determining the egg-laying reactions of the adults. The researches
of Diirken (1923) on the colours of butterfly pupae arc also
capable of alternative explanations, here in terms of unconscious
selection of predispositions, and/or of Dauermodifikationen.

There remain one or two results, such as that of Metalnikov
(1934) on immunity in waxmoths, and of Sladdcn and Hewer
(1938) on food-preferences in stick insects which seem prima facie
to demand a lamarckian explanation (but seep. 303 n.). However,
in view of the fate of other claims, and of the dieoretical diffi-
culties we shall discuss below, too much weight must not be
attached to such isolated cases.

Nor need we pay attention to the view advanced by certain
lamarckians, that the inherited effects of function or environ-
mental modification arc so slight that they cannot be detected
experimentally but require cumulative action through thousands
of generations to become obvious. Exceedingly minute differ-
ences can be detected by proper technique. The total failure of
sixty-nine generations of disuse to affect the eyes or the photo-
tropic responses of Drosophila, as shown by Payne (1911), is a good
example of the failure of disuse to produce lamarckian effects.

To plead the impossibility of detection is a counsel of despair.
It is also unscientific: the only scientific procedure would be to
refine technical methods until the postulated effects were capable
of detection. The experiment has nothing impossible about it
with pure-bred stocks and in a rapidly-breeding spedcs.

46 o evolution; the modern synthesis

It is, however, necessary to realize that important indirect
objections can be made to any lamarckian view. In the first
place there is die fact of mendelian recessivity. A recessive char-
acter can be rendered latent indefinitely by keeping the gene
concerned in the heterozygous condition; yet when the recessive
gene is allowed to unite with another like itself, die resultant
character is identical with that of pure-bred recessives in which
it has been manifested, and therefore exposed to environmental
stimuli, throughout.

An equally fundamental difficulty concerns all those almost
innumerable cases in which the two sexes differ in adaptive
characters of structure or behaviour. For we know with cer-
tainty that the genetic constitution, in the shape of the chromo-
somes, is distributed irrespective of sex. The chromosomes of a
sire will be distributed among his descendants of the second and
later generations according to the laws of chance, in a purely
random way, and equally among his male and female descendants
(a quantitative exception, but one irrelevant to our present
purpose, is provided by the distribution of the sex-chromosomes).
What lamarckian mechanism could ensure that the hereditary
efiects of functions confined to males are transmitted to male
descendants only.? The situation can just possibly be saved by
subsidiary hypotheses, but only at the cost of much superfluous
complexity, as the geocentric hypothesis was formally saved by
the doctrine of epicycles.

Apart from this, we find numerous cases where lamarckian
inheritance, even if it existed elsewhere, must be either impos-
sible or exceedingly restricted. Let us first take the case of the
higher mammals. These have their internal environment regu-
lated to an extraordinary degree of constancy. The temperature
of the blood and to a still higher degree its salt composition and
its acidity, are kept constant by elaborate special mechanisms.
The reproductive cells, like all other cells in the body, are exposed
to the internal environment supplied by the blood-stream. How
then can changes in the external environment be transmitted to
them.? The regulation of the internal environment provides an
effective shock-absorber for all the more obvious alterations

ADAPTATION AND SELECTION - ■ ' ' ^

which could occur' in the external eiiYiroiinieiit. Yet higher
mamnaals have evolved as rapidly and in as obviously adaptive
ways as any lower types in which this buffering does not exist.

The social Hymenoptera provide another natural experiment
of great interest, hi them, as is well known, the bulk of the work
of the colonies is carried on by neuter females, wliilc repro-'
duction is entrusted to the much less abundant full females and
the males. How is it possible on any lamarckian view to discover
a mechanism by which the special instincts and structures of the
workers have been evolved? They cannot transmit them in
reproduction, for they do not reproduce; and the males and
females do not practise the instincts nor possess the structures.
Attempts have been made to obviate the difficulty by pointing
to the fact that occasionally neuter females will lay unfertilized
eggs, so producing males. If, however, such occasional abnor-
malities of reproduction suffice to generate the elaborate special
characters of neuter ants and bees, then lamarckian transmission
operating through normal reproductive channels should have
such strong effects as to be detectable by the crudest experiment;
and this is certainly not the case.

Insects, indeed, provide a number of hard nuts for lamarckian
cracking. All liiglier insects emerge from the pupa into an adult
or imago stage, during which they never moult, and so arc
incapable either of total or local growth (save by mechanical
stretching of membranous parts of the exoskelcton), or of
alteration in the form of hard parts. Here again it seems all but
impossible to imagine any mechanism by which any modifi-
cation involving structural change in hard parts could be trans-
mitted. Indeed, such modifications cannot very well be pro-
duced at ail in the individual: thus the only lamarckian mechanism
conceivable is one by which a tendency or an attempt to alter
the striicture of hard parts would have its first visible effects in
the next or later generations! Yet adaptations of hard parts arc
striking in insects.

A very similar objection applies to mammalian teeth. These,
as is well known, exhibit remarkable adaptations to the type ot
food t>n winch they are normally used- Yet the only eficct of

462 evolution: THE MODERN SYNTHESIS

use upon them is medianical abrasion, tending to wear away
die structure which has been built up in the plastic stage when
the tooth is not used at all.

The origin of the general cryptic resemblance of animals to
the prevailing colour of their habitats, whether desert, open sea,
green foliage, tundra, or snow, has often been ascribed to a
direct effect of the environment. If this rather vague statement
means anything, it must imply that the characters in question
are either pure modifications, or have been genetically induced
by some form of lamarckism. Granted that there exist, notably
in insects, some cases of modification, we can now safely assert
that most of these characters are genetic. Some of these in their
turn may be merely correlated with physiological adaptations;
but there are some examples where we can show that selection
in favour of cryptic resemblance must have been the agency
at work.

In an earlier chapter we mentioned the case of dark subspecies
of Peromyscus inhabiting local lava-flows. Precisely similar
examples are known in birds, e.g. in the desert lark, Ammotnanes
deserti, of which the darkest and the palest subspecies live close
together in North Arabia, the one on black lava-desert, the other
on pale sand-deserts. This is cited by Meinertzhagen (1934), who
makes the pertinent comment that such cases of protective
resemblance to soil are largely confined to ground-loving birds.
Thus in the black Ahaggar desert, the Ammomanes zre very dark,
while the local babbler {Ar^ya fulvus buchamni) is even paler
than on the sand of the Sahara— presumably in relation to climate.
An even more striking case has recently been described for other
mice of another genus, Perognathus (J. E. HiU. 1939). In a valley
of southern New Mexico a black lava area of between 100 and
200 square miles exists quite close to an area of gleaming white
gypsum. P. iiitermedius exists on the lava beds in an almost black
form, while the representatives of P. apacPie on the gypsum area
arc nearly white. Both species have normal “mouse-coloured”
forms on neighbouring rocky areas. Other mammals, reptiles,
and insects from the two special areas show corresponding but
less extreme coljur modifications. Here again no climatic or

ADAPTATION AND SELECTION ^463

other influences capable of bringing about the colour-difierences
can be detected, and we are driven to conclude that the colour
is a protective adaptation originated by selection. If selection
can be effective in such cases, there is no reason to postulate any
lamarckian effect for any examples of general Cryptic resemblance.

Hovanitz (1940) cites a similar and very striking case from
the butterfly Oeneis chryxus, which in the Sierra Nevada exists
in two sharply-contrasted dark and pale subspecies or forms,
restricted to dark volcanic and pale granitic rocks respectively,
the two pure forms connected by dines extending over lo to
40 miles, where the rock-types are intermingled. Dark rock
outcrops in the granitic area below a certain size are inhabited
by Hght forms, being apparently too small to support a dark
population that can maintain itself against swamping by crossing.
Hovanitz is forced to the view that the two forms owe their
origin or at least their maintenance to selection, but rejects the
idea that this is exercised vii predators in relation to concealing
coloration. His objections may be profitably analysed. In the
first place, since the upper surface resembles the environmental
background much more closely than the lower, he states that
visuaJ selection by predators could only occur when the upper
surface is exposed, namely, in flight. However, his own photo-
graphs show a certain degree of difference in the lower surface.
Secondly, he states that when not in flight, they rest in “relative
darkness” between rocks, among herbage, etc., “where colour
is of no value”. This last statement is a mere assertion, as no
evidence is given as to possible predators in such situations. (In
other habitats, Lepidoptera are frequently captured when at rest.)
Finally, he states that almost the only possible predators are
two species of birds which only occasionally take insects, and
therefore cannot act selectively. In the first place, because his
search for predators has not been successful, that is no reason for
concluding that they do not exist. Cases must indeed be rare
where a small butterfly has no enemies. But further, he appears
to disregard the quantitative findings of students of the mathe-
matics of selection, such as Haldane (1932a) and R. A. Fisher
(1930a). A I per cent advantage — ^i.c. the average survival of

464 EVOIUTION: THE MODERN SYNTHESIS

loi members of one form as against 100 of another — ^would be
almost impossible to detect, yet it would promote an evolu-
tionary change of considerable rapidity, markedly modifying
the stock within a few hundred generations (p. 56).

Hovanitz also makes the general objection to the theory of
protective coloration that “the animals getting along best in
nature are those which are not ‘protected’ ” — a fallacy so hoary
that it hardly needs serious discussion (but see the general ana-
lysis, pp. 466 seq. ; and on hypertely, p. 484)- Most naturalists will
prefer to regard such a case as this as prima facie one of concealing
coloration until definite evidence to the contrary is produced.

Finally we may mention various special examples of pro-
tective resembiaiice and mimicry. The resemblance of certain
moths to birds’ droppings or of a stick insect to a stick cannot
very well be put down to the inheritance of environmental
modifications or the effects of use! In mimicry, the resemblance
of model to mimic is often achieved by way of a trick — a similar
effect is produced by quite a different mechanism. The “painting
in” of a waist on various beetles or bug mimics of ants is a good
example: numerous others may be found in Carpenter’s little
book on mimicry (Carpenter and Ford, 1933), or in the more
general work of Cott (1940).

These are some of the most striking cases in which a lamarckian
explanation cannot, it seems, apply. We have already seen (p. 38),
that, merely from the standpoint of logic and theory, most
adaptations or functional evolutionary changes could be inter-
preted equally readily on the basis of indirect control by selec-
tion as on that of direct control by environment and use. We
are therefore driven to ask why, when numerous adaptations
like those Just cited are shown to be incapable of lamarckian
explanation, we should postulate lamarckism to account for
the others, which are no different qua adaptations. To do so
would be to sin against the economy of hypothesis and demand
the application of William of Occam’s razor..

Thus we are driven back on to direct experimental proof,
and that, as wc have already set. forth, is meagre and confusing.
It is for these reasons that the majority of biologists, including

ADAPTATION AND SELECTION

the very great majority of those who have experience of
actual genetic work, repudiate lamarckism, or, at best, assign
to it a subsidiary and unimportant role in evolution. Even if
lamarckism be operative at all, it seems clear that some otlicr
mechanism must be invoked to account for the major part of
evolution.

Most biologists also look askance at orthogenesis, in its strict
sense, as implying an inevitable grinding out of results pre-
determined by some internal germinal clockwork. Tliis is too
much akin to vitalism and mysticism for their Hking: it removes
evolution out of the field of analysable phenomena; and it, too,
goes contrary to Occam’s razor in introducing a new and un-
explained mechanism when known agencies would suffice.
Furthermore, as R. A. Fisher has cogendy pointed out, the
implications of orthogenesis, like those of lamarckism, run
direedy counter to the observed fact that the great myority of
mutations are deleterious. In any event, as we shall see in a later
chapter (p. 506), the cases in which a true orthogenetic hypo-
thesis is demanded in preference to a selectionist one are very
few, and even in these few it may turn out that it is our ignorance
which is responsible for the lack of alternative explanations. As
set forth elsewhere (p. 516), numerous cases exist where evolu-
tionary potentiality is restricted; but these are quite distinct
from orthogenesis in the strict sense of a primary directive
agency in evolution.

Selection itself often produces an apparent orthogenetic effect.
This was realized by H. W. Bates over three-quarters of a century
ago in his classical paper on mimicry {1862), where he wrote
“the operation of selective agents gradually and steadily bringing
about the deceptive resemblance of a species to some other
definite object, produces the impression of their being some
innate principle in species which causes an advance of organiza-
tion in a special direction. It seems as though the proper variation
always arose in the species, and the mimicry were a predestined
goal”. However, these and the similar examples drawn from
paleontology (pp. 416, 515; 494) on analysis turn out to be much
better explicable on ;

466

E.¥0XUTI0N :' the- MGDERN'' 'synthesis; ,

purpose of adaptation is only a pseudo-teleology, so its apparent
inner direction is only a pseudo-orthogenesis.

7. THE ORIGIN OF ADAPTATIONS: NATURAL SELECTION

There remains natural selection. Before discussing some concrete
examples of selection at work to produce adaptation and ot
adaptations illustrating the work of natural selection, a ew
general points deserve to be made. In the first place there is the
aged yet apparently perennial fallacy that such-and-such an
arrangement cannot be adaptive, since related organisms^ can
and do exist without it. This is, quite franUy, nonsense. It is on
a par with saying that electric refrigerators are not^eful, because
many people, even among those who can afford the ex^nse,
manage to get on happily without them, or even that alphabets
and wheeled vehicles arc useless luxuries or accidents because
the negro and other human stocks never invented them.

There are in fact numerous possible explanations of such a
state of affairs. It may be that mutations in Aat direction did not
crop up, or were not availRble before mutations in some other
direction set the stock specializing along other lines; it may be
that there are differences in the genetic make-up or the environ-
ment of the two forms, as yet undetected by us, which make
such an adaptation less advantageous to one than to the other.

All that natural selection can ensure is survival. It does not
ensure progress, or maximum advantage, or any other ideal
state of affairs. Its results, in point of fact, are closely akin to
those of commercial business. In business, what gets across-
ie. is sold — is what can be sold at a profit, not by any means
necessarily what is best fitted to meet the real needs' of indivi-
duals or of the community. The reason for the failure of a
commodity to be sold may be lack of purchasing power in the
community as much as poor quahty, or lack of persuasive (and
not necessarily truthful) advertising as much as inefficient pro-
duction methods.

In the same way a species or a type may survive by deceiving
its enemies with a fraudulent imitation of a nauseous form just

adaptatign and selection 467

as well as by some improvement in digestion or reproduction,
by degenerate and destructive parasitism as much as by increased
intelligence. There still exist those who, even while rejecting
the view of Paley and his school that adaptation is a proof of
divine design, continue to approach evolution in a rather rever-
ential attitude and to attach some sort of mOrd flavour to natural
selection. They should be reminded of adaptations such as those
by which the ant-parasite Lomechusa obtains its food, or the
orchid Cryptostilis ensures its reproduction. .Loffiec/iHSu produces
a substance which the ants so dote upon tliat they not only feed
the adult beetle in return, but allow its grub to devour their
own larvae— a sacrifice to a gin-producing moloch (Wheeler,
1910) ; Cryptostilis practises an ingenious variety of prostitution :
by resembling the females of a fly both in form and in odour,
it induces the males to attempt copulation with its flowers, thus
securing its own pollination (Coleman, 1927).

We should finally remember that the incidence of selection
is different for rare and for abundant species, aiid that an adap-
tation forcibly promoted by intraspecific selection in an abundant
species might have little or no biological value when worked
upon by interspecific selection in its rarer relatives.

It is another fallacy to imagine that because the major elimina-
tion of individuals occurs, say, in early Ufe, that therefore selection
cannot act with any intensity on a phase of mmimum numbers,
say the adult stage. It has, for instance, been argued that because
the main elimination of butterflies takes place by parasitization
or enemy attack during the larval stage, therefore elimination
of the imagines by birds or other enemies can have no appreciable
selective effect, and therefore any protective or warning or
mimetic colouring which they exhibit caimot have any adaptive
significance. But selection heed not act with equal intensity at
ail stages of the life-cycle: even, if it should be more intcn.se in
early hfe (and much early mortality appears to be accidental),
it could still produce effects on adult characters (see A. J. Nichol-
son, 1927).

The same argument appHes to adaptive colouring shown in
the larval stage. Even if this has no effect in protecting the larvae

4.68 EVOtUTION- THE MODERN SYNTHESIS

from parasitization, it will have selective value if it protects
from attack by other enemies. Granted that on the average
90 per cent of larvae will in any case succumb to parasites, selec-
tion can clearly act in other ways on the remainiag 10 per cent,
just because they are the survivors. In general, selection may
promote highly specialized adaptations not only in any particular
organ or function, but at any particular phase of the life-history.
The elaborate pelagic specializations of many invertebrate larvae
at once come to mind, or the adaptations of seeds. Salisbury
(1929) cites an interesting case of juvenile adaptation: most
plant species, eiren if lightrdemanding forms, show greater shade-
tolerance in early life, which militates against suppression by
shading in the crowded conditions soon after germination.

However, as Professor Salisbury points out in a letter, since
the adult phase follows the juvenile in time, many adult charac-
ters may well be non-selective qua adult characters, but merely
consequential results of juvenile adaptations. Some cases of this
sort are discussed later (p. 525).

It is, after all, the adults which reproduce, and a i per cent
advantage of one adult type over another will have precisely
the same selective effect whether the adults represent ten, one,
or one-tenth of i per cent of the number of fertiHzed eggs
originally produced. The same applies to those plants in which
the main elimination occurs during the seedling stage. Selection,
in fact, can and does operate equally effectively at any stage of
the life-cycle, though it will operate in entirely different ways
at one time and another. Further, elimination is far from being
the only tool with which selection operates. Differential fertility
of the survivors is also important, and in man and many plants
is probably the more influential.

There is finally the experimental demonstration of selection.
Wc have referred to this on p. 120; see also p. 414 for ihe summary
of such work on adaptive coloration given by Cott (1940),
Here wc may cite a further piece of work.

Popliam (1941) has made a careful investigation of the bip-
logical significance of the variation in colour (measured in terms
of shade of grey) in various water-boatmen (Corixidac). The

adaptation and SIMUCTION A('9

.nimals tend to resemble the backgrounds of the ponds where
tCtc found. This is due partly to habitat-selection: animals
confined in surroundings markedly different in backgroun loin
ibeir own shade become restless and leave to seek other waters.-
Secondly, it is due to developmental colour-adaptation, tic
nvniphal and adult shade approximating to that of the si -
oCings in which they have lived in the previous instar (though
• ■ * itiQt'ir there is no power of colour-adjust niciit).

AnTImlfy, it is ht » seteion, prc<btors (riidd, Scar&te

“ 'rTfc lirTX

teLd” aitira* ^ ^

to selection, a number of tosting tcsulB were
1^' For one thine, two fairly siniilat colour-vancncs for
S ire w“ n.ar£d differential predation when one of
1 resembled the background, were equally attacked

*^hm the background was markedly different from both. It is
when th g relative diftcrcncc of the two

thus, as woul ^ absolute difference between

as expected, a decrease in
them, winch acts sclcc y g ^ the advantage of

t implies a sclf-rc^Noty

Ih:^ ^ regards predator-prey balance, pretemon eon-

A'tLrrTe W roXonXhy *0

Ti^Stively the selection in certain circumstances was very
Quantitatively, t _ . ^j^^h there were employed

mtense. E.g. in « ^ ^hade as the background

rp“fo3”) and differing from it by one colour-sta..dard

Vm^^«etcd-), the relevant results were a, follows.

Insects eaten

Selective advantage

470 evolution: the modern synthesis

In another set of experiments, three types of insects were
used, differing from each other by one colour-standard, and used
against backgrounds of various standards. The results show the
variation in intensity of selection with change in the relative
difference of coloration between insects and background.

Per cent eaten of insects,
of colour-standards: —

Selective advantage'

(x 4- 1) (x + 2) (x + 2)

Differences

A 1

from

(x+ 1

backgroun

d {m

colour-standard

units)

4. 5. 6

All approximately equal

3 . 4,5

1*29

1*00

2, 3 . 4

1*19

1*32

1*28

1,2,3

3-27

4-82

iH7

A further set of experimaits was carried out with species of
water-boatmen of different sizes. It was found that the predator
used, the rudd, is almost entirely restricted to those of a certain
intermediate size. Large forms were difficult to capture (14 per
cent taken as against 86 per cent of a medium-sized species)
while small species were apparently not noticed at all. This
illustrates the point made on p. 280, that a predator must be
adapted to its prey in size as in other respects.

Selective advantage is here, in certain conditions, very large.
But we must remember that an advantage which it would be
extremely difficult to demonstrate experimentally, say of i per
cent, would have an effect which, biologically speaking, would
be rapid (see p. 56).

Various cases where a selective balance is involved show as
forcibly as any laboratory experiment the strength of selection-
pressure. We have referred to some of these in the section on
polymorphism (p. 96). The best of all (see p. 93), is probably
that of industrial melanism. Ford (1940b) has recently shown
that in unfavourable conditions (feeding only on alternate days)
the dominant melanic form of the moth Boarmia repandata has
a selective advantage of nearly 2 to i (52 Idacks : 31 normals
surviving to the imago stage where equality was expected).

ADAPTATION AND SELECTION 471

Even where opdmuoi food-conditions were provided, the ratio
was loi : 91. Yet in spite of this enormous constitutional advan-
tage of the melanics, the selective advantage conferred by cryptic
colouring on the hon-melanics has prevented their replacement
by melanics in all noil-industrial areas. Mr. Ford informs me that
in another case (not yet fully analysed), the melanic form is more
cold-resistant. Yet it has not managed to oust the cryptic form,
even in the extreme north of Scodand, far to the north of the
industrial regions where it has become the type.

An elaborate and large-scale demonstration of selection in
action, has been given by the work of Quayle (1938) on the
gradual development, by various scale-insect pests of citrus
fruits, of a high degree of genetic resistance to the hydrocyanic
acid used to try to kill them. As long ago as 1914 Quayle’s
attention was drawn to the unsatisfactory results from tent
fumigation of lemon trees against red scale {Aonidiella durantii)
in the Corona district of Cahfomia. In most localities it was not
then necessary to repeat fumigation for two, three, or even four
years. At Corona, however, neither increased dosage nor repeti-
tion of fumigation every year or even every six months was
effective.

Controlled experiments were later carried out in which the
scales from different areas were grown on the same tree and
exposed to different concentrations of gas in the same chamber.
The results showed that whereas in insects from many localities
the normal dosage was reasonably effective, and no scales sur-
vived a 50 per cent increase of dosage, in those from the resistant
areas, about five times as many survived normal dosage and
almost as many survived the increased dosage as survived the
normal one in the case of non-resistant strains.

In 1915 evidence turned up of a resistant local strain of the
black scale {Saissetia oleae), and since ihen the area of resistance
has spread and the degree of resistance has been increased. In
1925 a resistant stra in of the citricola scale. Coccus pseudomagno-,
liarum, was first observed. Prior to this date, fumigators had
guaranteed their work with this pest and offered a second fumi-
gation free if the first proved unsatisfactory. In the next few

EVOLUTION : THE MODERN SYNTHESIS

473

years, the area of resistance spread rapidly, and the highest
dosages compatible with the health of the trees failed to give
satisfactory results, with the result that fumigation could not be
guarantee^ and was eventually abandoned in favour of spraying.
Controlled experiments showed that a dose four times the
“danger dose” for trees was needed to kill all the resistant insects.
In another experiment all insects of a non-resistant strain were
killed by sixty minutes’ exposure to 0.05 per cent HCN. But
after sixty minutes’ exposure to a sixfold increase of gas (0. 3
per cent), many insects of a resistant strain were alive and a few
survived ninety minutes. In general the resistant strain, in con-
centrations which killed 60 to 100 per cent of non-resistant
strains, proved from two to four times more resistant.

Scale insects are not the only forms to show diis phenomenon.
Hough (e.g. 1934) experimentally proved not only that strains
of codling moth {Cydia pomonella) from diiSerent areas differ
markedly in the capacity of their larvae to enter apples sprayed
with lead arsenate, but that, when the strain is raised on freshly
sprayed fruit in the laboratory, the percentage of larvae capable
of this increases from generation to generation.

Resistance in red scale is genetic (see Dickson, 1941) and it
remain unaltered after many generations in the laboratory. An
interesting fact is that the resistant strain of red scale has shown
itself more resistant to various other toxic substances, to which
it has not been exposed in the orchards, e.g. to the fumigants
methyl bromide and ethylene oxide, and to oil sprays. It is
also probable that it shows greater ability to withstand desicca-
tion. Thus its newly evolved resistance appears to be a general
rather than a specific one: the same is true of the codling moth.

Quayle concludes that the resistant strains have developed
locally, as a result of intense selection due to the fumigation
methods in vogue. When, as appears usual, they have developed
earlier in some locaKties than in others, this is presumably due to
the availability of actual or potential variance of the right type,
or of new mutations in the right direction. In all cases the area
inhabited by resistant strains has rapidly increased. Quayle gives
reasons for thinking that this is in the main due to the rapid

ADAPTATION AND SELECTION 473

spread of local resistant types, as soon as these are available
through mutation or recombination, rather than to immigration
of the resistant forms from the localities where they first appeared.
Quayle further points out that the standard fumigation dosage
in California, in non-resistoit as well as resistant areas, is now
much higher than originally. “The schedules have been revised
several times and always upwards. It is interesting to note that
in Australia, South Africa, and Palestine, countries much younger
than California in fum^ation practice, the dosage used against
the same insect is much lower than in California.”

This large-scale experiment with its laboratory controls is of
great interest in showing that intense selection may be very
effective in brit^h^g about important changes, and in giving
indications as to the rate at which the process can operate.

As regards the intensity of selection operating in nature,
R. A. Fisher (1939) has been able to calculate the selection
operating against {a) homozygosity as against heterozygosity of
the various single dominants giving the numerous colour-patterns
other than the normal or basic one, (b) combinations of two of
these dominants, in the grasshopper Paratettix texanus (cf. p. 99).

The selection against homozygous single dominants varies
from about a 7 per cent to a 14 per cent disadvantage, while the
elimination of double dominants is estimated to be not less than
40 per cent in each generation. This Fisher considers points to
“powerful and variable ecological causes of elimination”, whereas
the selection in favour of single-gene heterozygotes is probably
to be accounted for solely in terms of viabOily differences.

In any case, if we repudiate creationism, divine or vitalistic
guidance, and the extremer forms of orthogenesis, as originators
of adaptation, we must (unless we confess total ignorance and
abandon for the time any attempts at explanation) invoke natural
selection — or at any rate must do so whenever an adaptive
structure obviously involves a number of separate characters,
and therefore demands a number of separate steps for its origin.
A one-character, single-step adaptation might clearly be the
result of mutation; once the mutation had taken place, it would
be preserved by natural selection, but selection would have

474 evolution: the modern synthesis

played no part in its origin. But when two or more step are
necessary, it becomes inconceivable that they shall have origi. ated
simultaneously. The first mutation must have been spread through
the population by selection before the second could be combined
with it, the combination of the first two in turn selected before
the third could be added, and so on with each successive step.
The improbabihty of an origin in which selection has not played
a part becomes larger with each new step.

Most adaptations clearly involve many separate steps or
characters: one need only think of the detailed resemblance of
a close mimic to its mockl, the flying qualities of a bird’s wing,
the streamlining of secondary aquatics like ichthyosaurs or
whales. When we can study actual adaptive evolution with the
aid of fossils, as with the hooves of horses or the molar teeth of
elephants, we find that it is steadily directional over tens of
millions of years, and must therefore have involved a very large
number of steps. The improbability is therefore enormous that
such progressive adaptations can have arisen without the opera-
tion of some agency which can gradually accumulate and com-
bine a number of contributory changes: and natural selection
is the only such agency tihat we know. In such cases it is especially
evident that what is selected is not a particular gene, but a whole
complex of genes hi regard to their combined interacting effect
(see Sewall Wright, 1939, who has an interesting discussion of
the systems of mating, breeding, and selection best suited to
obtaining results with various types of genes and gene-combina-
tions affecting a given pharacter).

R. A. Fisher has aptly said that natural selection is a mechanism
for generating a high degree of improbability. This is in a sense
a paradox, since in nature adaptations are the rule, and therefore
probable. But the phrase expresses epigrammatically the important
fact that natural selection achieves its results by giving prob-
abihty to otherwise highly improbable combinations — and “in
the teeth of a storm of adverse mutations” (R. A. Fisher, 1932).

This is an important principle, not only for the conclusion that
adaptations as seen in nature demand natural selection to explain
their origin, but also for its bearing on the “argument from

ADAPTATION AND SELECTION 475

improbability”, used by many anti-Darwinkns against Dar-
winism in general. Bergson has employed this with regard to
the origin of the eye. Haldane {1932(1) and others, however,
have pointed out that a gradual improvement of the visual
mechanism from pigment-spot to fully-developed eye is to be
expected, and that the parallel development in vertebrates and
cephalopods of eyes with lenses is, on the basis of the laws of
optics, not in the least unlikely. Indeed, on more general grounds,
the properties of natural selection entirely nullify the argument
from improbability in this and other cases.

Thus T. H. Morgan and Hogben have asserted that natural
selection is seen, in the light of modem genetics, to be essentially
destructive: in the absence of natural selection, all the known
forms of life would exist, and in addition a vast assemblage of
other types which have been destroyed by selection. Though
both have now adopted a much more selectionist standpoint,
these past views must be refuted as anti-selectionists still often
cite them.

T. H. Morgan (1932, p. 130) writes: “If all the new
mutant types that have ever appeared had survived and left
offspring like themselves, we should find all the kinds of animals
and plants now present, and countless others.” The catch here
is in the if; and the answer, of course, is that every type imme-
diately ancestral to a mutant has been brought into existence
only with the aid of selection (see also Hogben, 1930, p. 181).

In point of fact the general thesis is entirely untrue. It is on
a par with saying that we should expect the walls of a room to
collapse on occasion owing to all the molecules of gas inside
the room moving simultaneously in one direction. Both arc of
course only improbabilities — ^but they arc improbabiHtics of such
a fantastically high order as to be in fact entirely ruled out.
Each single existing species is the product of a long series of
selected mutations. To produce such adapted types by chance
recombination in the absence of selection would require a total
assemblage of organisms that would more than fill die universe,
and overrun astronomical time.

It should further be remembered that the degree of adaptive
specialization is correlated witli intensity of selection-pressure.

476 evolution: the MODfifiN synthesis

Fisewhece (p. 426) we have noted how, in the balance between
the opposed adaptive tendencies towards cryptic coloration and
display coloration in birds, the degree of development of display
(epigamic) adaptations is directly proportional to the repro-
ductive advantage it confers upon an individual male. The
greater abundance and development of cryptic and aposematic
adaptations to be found in the tropics, where selection-pressure
is highest, is also to be noted (see p. 448).

The converse of this positive correlation is the tendency of
originally adaptive stmctures or fufictions to degenerate in the
absence of further selection-pressure in their favour. We have
spoken of this in relation to the eyes and pigmentation of cave
animals (p. 453), but the fact is one of the commonplaces of
evolutionary biology, e.g. in parasites. The vestigial wings of
ratite birds provide an excellent example. These are in all cases
degenerate as regards the adaptations needed for flight. Where,
however, they are employed in epigamic display as in the
ostriches [StrutUo) or the rheas {Rhea), they remain of consider-
able size; but where this further function is absent, as in the
emus {Drmaet 4 s) and cassowaries {Cemarius), they are reduced
to vestiges. This tendency towards degeneration of useless
structures — ^i.e. those on which selection-pressure is no longer
maintainedr— is, as we have seen (p. 455), automatic in most
organisms, owing to the accumulation of small degenerative
mutations that throw the dehcate mechanism of adaptation out
of gear. This may be further generalized in terms of gene-effects
(Wright, 1929). Most genes have multiple effects. Organs under
direct selection will be modified by a system of genes; but the
genes of such a polygenic system will also have secondary effects
on “indifferent” organs, and most of these secondary effects wiU
tend to promote degeneration in size or function. Further, when
two itnked polygenic systems (p. 67) are lodged in the same
chromosome or chromosomes, and selection is acting to alter die
main character controlled by one system, while that controlled
by the other is useless, the resultant recombination will ‘ ‘break up ’ ’
the useless character; in virtue of the tendency of random change
to be towards decreased efficiency, this also will promote de-
generation.

ADAPTATION AND SELECTION 477

None of this reasoning, however, should apply in the case of
organisms which do not practise outcrossing. Here, the recom-
bination of “loss” mutations is impossible, and thus degeneration
should be exceedingly slow. Furthermore, since many loss muta-
tions need recombinational buffering (p. 67) to survive, they will
be automatically eliminated where recombination is impossible.
The result should be the persistence of originally adaptive but now
functionless structures. The natural place to look for such “relict
adaptations” is the floral mechanisms of plant species which have
wholly abandoned outcrossing.

At first sight there would appear to be numerous examples of
this. For instance, in various Compositae, such as dandelions
(Taraxacum) and hawkweeds (Hieracium) there exist a number of
forms which, in spite of producing all their seed by obligatory
apomixis, continue to form showy flower-heads, obviously
adapted to attract insects. However, the persistence of these erst-
while adaptations may be due to the short time elapsed since the
change to apomixis. On the other hand, in Taraxacum Dr. TurriU
informs me that apomixis very probably dates back at least 10,000
years.

A more serious objection is the existence of numerous “corre-
lated chs^racters” of the capitulum which still have functional
significance. Various parts of the mechanism provide the de-
velopmental scaffolding for the adaptive pappus; the ray florets
stiU play a protective role during the night closure of the head,
though this protection itself is perhaps a relict adaptation as it
probably concerns the pollen. However, such considerations
would not apply to obHgate apomicts in grasses, where the relict
floral mechanism was adapted to anemophilous cross-polKnation,
nor to the vegetatively reproducing coral-root, Dentaria bulbifera,
which still makes the unnecessary gesture of producing obviously
entomophilous flowers without any apparent subsidiary function.

Obligatory self-pollination should produce the same result.
Here the difficulty is to find satisfactory examples, since in most
cases some outcrossing still occurs. Thus the orchis Epipactis tepto-
chila is normally self-pollinating, but cross-pollination can occur
during a brief period. The closely allied E. latifoUa is exclusively
cross-pollinated (Godfery, M. J., 1933, Monograph of British

478 evolution: the modern synthesis

Orchiciaceae, Cambridge). However, Dr. Madier informs me
that, in Britain at least, the tomato [Sohmm lysopersicum) shows
no cross-pollination (save in one anomalous variety) ; yet its obvi-
ously entomophilous flowers persist. In some cereal strains, die
frequency of cross-pollination is so low (only 2 per cent) that it
should enormously reduce the speed of degeneration.

There is thus a prima facie case for the penistence of ‘TeHct
adaptations” whenever cross-breeding is absent (and perhaps
when markedly reduced), but more investigation is required for
full confirmation (Sec Huxley, 1942, Nature J49; 687).

8. ADAPTATION AND SELECTION NOT NECESSARILY
BENEHOAL TO THE SPEOES

So far, we have been discussing adaptation more or less in vacuo.
We roust now draw attention to the important fact that it will
have different effects according to the type of selection operating.

This is best illustrated by the distinction between interspecific
and intraspecific selection. In one sense, almost aU selection is
mtraspecific, in that it operates by favouring certain types within
the species at the expense of other types. The only exceptions
would be when species spread or become extinct as wholes.
The former occurs with such species as are produced abruptly,
e.g. by allopolyploidy after hybridization. The latter occurs
when no strains within a species are capable of adjusting them-
selves to a change of climate or to the arrival of new competitors
or enemies. Selection in such cases no longer operates by any
diSerential action between different strains, and the whole species
spreads or disappears in competition widi other species.

The term intraspecific selection can, however, properly be
used in a more restricted sense, to denote selection concerned
only with the relations of members of one species. On the same
basis, interspecific selection is then selection which is ultimately
concerned with the environment or with other species. Thus
selection for speed in an ungulate wiU operate intraspedfically
in the broad sense, hut k directed interspecificaUy in being con-
cerned with escape from predators. Similarly selection for cold-
resistance in a period of decreasing temperature is directed

ADAPTATION AND SELECTION 479

environmentally, and may favour the entire species in competition
with others. But selection for striking epigamic plumage in male
birds is directed intraspecificaUy, in being concerned with the
advantage of one male over another in reproduction. It would
thus be more correct to speak of selection concerned with intra-
or interspecific adaptation; however, it is more convenient to
use the terms in the sense I have just outlined.

We have already discussed intraspecific selection briefly in
relation to the numerical abundance of species (p. 34). In scarce
species, competition will be more with other species and selec-
tion will be related more directly to the environment, wliile. in
abundant species there will be more competition between indi-
viduals of the species itself. Of course inter- and ititraspecific
selection will often overlap and be combined; but the intensity
of one or the other component may vary very greatly.

An interesting type of selection which is in a certain sense
intermediate between interspecific and intraspccific, may occur
in forms which exist in numerous and relatively isolated local
populations, particularly if the local populations are subject to
large fluctuations in numbers. In such cases (Wright, 19406) a
local population may “arrive at adaptations tliat turn out to
have general, instead of merely local, value, and which thus
may tend to displace all other local strains by ... excess
migration”. Wright calls this intergroup selection. When this
operates, groups compete qua groups, on the basis of elaborate
gene-combinations restricted to the separate groups. It is prob-
able that this type of evolution has played a considerable role
in some kinds of species : cf. Sumner (1932, p. 84) for Peromyscus.

Intergroup selection, however, may operate betv^ecn groups
with a functional basis as well as between those with a regional
basis (local populations). Iiitcrgroup selection of this sort we
may perhaps call social selection, since it will encourage the
gregarious instinct and social organization of all kinds. As AUec
(1938) has recently stressed with the aid of a wealth of examples,
the bases for social life in animals arc deep and widespread.
There exist numerous cases where it has been experimentally
shown that aggregations of a certain size enjoy various physio-

48o EVottmoN: the modern synthesis

logical advant^es over single individuals. Once that occurs,
selection will encourage behaviour making for aggregation and
the aggregation itself will become a target for selection.

In a later paper (1940) Alice develops this theme further.
He shows that when degree of crowding is plotted against
efficiency for a large number of functions, the resultant curves
are of two sharply distinct types. In the first type (which I
suggest might be distinguished as unit-selective, since selection
feUs on the unit individual), the performance has optimum
efficiency of lowest population density (e.g. a single pair for
maximum fertility per pair in various insects). But in the second
(which perhaps could be called group-selective, not because there
are more selective factors, but because the group of many indi-
viduals becomes a target for selection), there is a phase of
“undercrowding”, during which the efficiency of the function
increases with population density, finally reaching a peak and
then descending in a phase of overcrowding. A special case is the
reproductive advantage conferred by size of colony in colonial-
nesting birds (Darling, 1939; Vesey-Fitzgerald, 1941, p. 535;
J. Fisher and Waterston, 1941; and cf. p. 103).

Processes of this type will of course give curves differing in
shape, slope, and so forth, and will have correspondingly different
results. Wherever such a curve occurs, it means that an aggre-
gation near the peak value will constitute “a supraindividual
unity on which natural selection can act. . . . Such low or
feeble social units may be poorly integrated, but still possess
demonstrable survival value”; and out of such primitive group-
ings, intergroup social selection can evolve such specialized
group-units as the ant or termite colony.

Finally, since processes giving curves of the multiselective
type have been discovered in every major group of animals, it
becomes clear that social selection will be widespread, and that
“sociality is seen to be a phenomenon whose potentialities are
as inherent in Hving protoplasm as are the potentialities of
destructive competition”.

In general, the intraspecific type of selection is much com-
moner than is generally supposed. Thus to drink of natural

ADAPTATION AND SELECTION 481

selection as first and foremost a direct struggle with enemies or
with the elusive qualities of prey is a fallacy. An equally important
feature of the struggle for existence is the competition of members
of the same species for the means of subsistence and for repro-
duction. Surprise has been expressed by some biologists at the
fact that in New Zealand, domestic pigs which have become
feral have, in spite of the absence of predatory enemies, reverted
to something like the wild type; but in competition for food
and reproduction the leaner and more active wild type must
clearly have a strong relative advantage over the fatter and more
sluggish domestic forms, so that reverse mutations or reversionary
recombinations will be favoured by selection.

Elsewhere (p. 426) we consider other examples of intraspecific
selection. Sometimes the competition is restricted to individuals
of one sex, as in intrascxual selection; sometimes to individuals
of a single Utter, as in the intrauterine selection of mammals
(p. 525). Again it may be especially intense at a certain period
of life, as is the competition for light and space between the
seedlings of many higher plants. Another example from plants
concerns the competition between the haploid male plants pro-
duced by the pollen-grains. Genetic research has shown that
these may be afi'ccted in various ways, including the rapidity of
their growth down the style, by the genes they bear. As a result
of this, certatim, or a “struggle for fertilization” between
genetically different types of pollen-grain, may and often does
occur, and genes which induce rapid growth of pollen-tubes
will often be at a premium. Nothing of die sort, however, appears
to take place in higher animals. The only known exception is the
gene described by Gershenson (1928) in Drosophila, widi ledial
effects only on Y-bcaring sperms. There is also the alleged dif&r-
cntial activity of die two types of sperms in forms with male
heterogamety; if this be a fact, it is probably due to some effect
of differential size, the male-determining appearing to have in
many cases a smaller head.

Even in most of the relations between a species with its enemies,
competition is intraspcdfic. Normally, a certain number of indi-
viduals arc bound to be killed; when so, the main pressure of

482 evolution: the modern synthesis

selection is directed to keeping an individual out of the category
of the relatively unprotected, where it wiU be an almost certain
victim, into that of the well-protected where at least it has an
even chance of survival. Any improvement in the protection of
some individuals will lead to the bulk of the population being
placed at a disadvantage, so that they vnll once more come
under selection-pressure. Such considerations will apply to speed
in escape, cryptic and mimetic resemblance, and many other
adaptations against predators.

In a different sphere, most competition within civilized human
societies is between individuals. The difference of course is that
success in this competition is not biological, measured by in-
creased survival to later generations, but social, consisting of
monetary and other satisfactions; in fact social and biological
success are usually inversely correlated.

Artificial selection is clearly intragroup in its methods. Thus
racehorses are selected for reproduction almost entirely on tlie
basis of their individual performances. In most domestic forms,
however, once marked breed characteristics have been estab-
lished, intergroup (interbreed) competition may operate, and
reduce or wholly eliminate certain types.

The dependence of the results of selection on the type of
competition prevailing is well seen in the case of the social
hymenoptera, such as honey-bees, wasps, and ants, where repro-
ductive specialization prevails, and therefore the extinction of
individual neuters can have no efiect on the constitution of later
generations, provided that the community survives (see p. 480).*

Haldane (1932a) has demonstrated that only m such a society,
which practises reproductive specialization, so that most of the
individuals are neuters, can very pronounced altruistic instincts be
evolved, of a type which “are valuable to society, but shorten die
lives of their individual possessors”. Thus unless we drastically alter
the ordering of our own reproduction, there is no hope of making
the human spccicsmuch more innately altruistic than it is at present.

* As Wdsmann early pointed out (sec discussion in Emerson, 1939), selec-
tion of this type will become more effective as the number of reprodiictives in
a colony is reduced — hence the single-queen condition in most termites and
social hymenoptera.

ADAPTATION AND SELECTION 483

The existence of intraspccific selection, i.e. selection between
genetically different types within a species, enables us to expose
another widespread fallacy — ^namely, that natural selection and
the adaptations that it promotes must be for the good of the
species as a whole, for the good of the evolving type pursuing a
long-range trend, for the good of the group undergoing adaptive
radiation, or even that it must promote constant evolutionary
progress. In actual fact we find that intraspecific selection fre-
quently leads to results which are mainly or wholly useless to
Ac species or type as a whole. Thus the protection afforded by
a cryptic or a mimetic resemblance of moderate accuracy might
speedily approach Ae limit so far as its value to Ae species is
concerned, if Acre were any way in which selection could be
restricted to effects on Ae species as a species. But as a matter of
fact selection acts via individuals, and this intraspecific compe-
tition between inAviduals will often lead to the process of
adaptation being continued until almost increAbly detailed
resemblances are reached. The perfection of the resemblance of
Kallima to a dead leaf is one of the marvels of nature; not the least
marvellous aspect it is that it is of no value to the species as a
whole (sec p. 427).

A. J. Nicholson (1933) has pointed out how advantages
operaAig at one stage of the life-history may be compensated
for by increased mortality in other stages, so that the species
docs not benefit as a whole. Thus in most Lepidoptera a cryptic
pattern favouring survival of adults will result in more larvae,
which in turn will permit a disproportionate increase in para-
sitoid infection, thus bringing down Ac number of adults again.
Wherever this balance ofelimmation as between stages is approxi-
mately self-regulating, factors affecting it will be over-riding as
regards interspecific selection, while selection for other characters
must be intraspccific. (In very imfavourable conditions wiA
much reduced adult numbers, the cryptic pattern might become
valuable for Ae species as a whole.)

In such examples, Ae adaptation is at least not deleterious. In
other cases, however, it may lead to deleterious results. This is
perhaps-especiaUy true of selection which is not only intraspccific

484 evolution: the mopekn synthesis

—confined to competition between members of the same speaes
—but also intrasexual— confined to competition between mem-
bers of the same sex of the same species. When polyganyr or
promisiscuity prevaUs, the selective advmtage conferred by
characters promoting success in matmg will be extremclv high
(p. 427): accordingly in such forms we meet with male cpigamic
characters of the most bizarre sort which, while advantaging tlicir
possessor in the struggle for reproduction, must be a real handi-
rap in the struggle for individual existence. The tram ot the
peacock, the tail of the argus pheasant, the plumes of certam
birds of paradise, the horns and antlers of certain ungulates, arc
obvious examples. In such cases of course a bailee will even-
tually be struck at wliich the favourable effects shghtly outweigh
the unfavourable; but here again extinction may be the fate ot
such precariously-balanced organisms if the conditions change

too rapidly (sec Huxley, 1938^ and b). i t u

We may, however, go further and suggest with Haidmc
(i932fl) that intraspecific selection is on the whole a biological
evil The effects of competition between adults of the sainc
spedes probably, in his words, “render the species as a whole
less successful in coping with its environment. No doubt weak-
lings are weeded out, but so diey would be in competition with
the environment. And the special adaptations favoured by intra-
spcdfic competitions divert a certain amount of cner^from
Other functions, just as armaments, subsidies and tariffs, the
organs of international competition, absorb a proportion of the
national wealth which many believe might be better employed .

Intraspecific competition among anemophilous plants has led,
it seems, to a real overproduction of pollen; among male
mammals to unwieldy size as in sea-elephants, or to over-
developed weapons and dircat-organs as in deer and various
horned groups; among parasites to their often monstrous exag-
gerations of fertility and complications of reproductive cycle.

There can be little doubt that die apparent orthogenesis which
pushes groups ever further along their line of evolution until,
as with size in some mcsozoic reptiles and armour in others, they
arc balanced precariously upon the edge of extinction (p. 506),

ADAPTATION AND SELECTION 485

is due, especially in its later stages, to the hypertcly induced by
intraspccific competition.

This conclusion is of far-reaching importance. It disposes of
the notion, so assiduously rationalized by the militarists in one
way and by the laisser-faire economists in another, that all man
need to do to achieve further progressive cvolutioit is to adopt
the most thoroughgoing competition: the more rutliless the
competition, the more efficacious the selection, and accordingly
the better the results. . . . But we now realize that the results
of selection are by no means necessarily “good”, from the point
of view cither of the species or of the progressive evolution of
life. They may be neutral, they may be a dangerous balance of
useful and harmful, or they may be definitely deleterious.

Natural selection, in fact, though like the mills of God in
grinding slowly and grinding small, has few other attributes
that a civilized religion would call Divine. It is efficient in its
way — at the price of extreme slowness and extreme cruelty.
But it is blind and mechanical; and accordingly its products are
just as likely to be aesthetically, morally, or intellectually repul-
sive to us as they arc to be attractive. We need only think of the
ugliness of Sacmlim or a bladder-worm, the stupidity of a rhinoceros
or a stegosaur, the horror of a female mantis devouring its mate or
a brood of ichneumon-flies slowly eating out a caterpillar.

Both specialized and progressive improvements arc mere by-
products of its action, and are the exceptions rather than the
rule. For the statesman or the eugenist to copy its methods is
both fooUsh and wicked. As well might the electrical engineer
copy the methods of the lightning or the hcating-cnginccr those
of the volcano. It indubitably behoves us to study the methods
of natural selection, but this will be to discover how to modify
and control them in new ways and, very definitely, to see what
to avoid. Not only is natural selection not the instrument of a
God’s sublime purpose; it is not even the best mechanism for
achieving evolutionary progress. An important step towards a
rational applied biology will be the full analysis of the various
modes of operation of selection with a view to its eventual
control and its intensification for our own purposes.

CHAPTER 9

EVOLUTIONARY TRENDS

I* Trends in adaptive radiation p* 4^6

2. The selective determiiiatioE of adaptive trends .... p. 494

3 , The apparent orthogenesis of adaptive trends . . . , p. 497

4 , Non-adaptivc trends and orthogenesis p. 5^4

5. The restriction of variation . . . p. 516

6 , Consequential evolution: the consequences of differential

development p* 5^5

7 . Other consequential evolutionary trends ..... . p. 543

I. TRENDS IN ADAPTIVE RADIATION

We liave now to consider long-range evolutionary trends. The
primary evidence on these comes from continuous fossil series,
but incomplete or even fragmentary series may often be satis-
factorily completed by the use of indirea evidence from com-
parative anatomy and embryology, and the indirect evidence
may supplement the direct by showing us, to a considerable
degree of probability, with what physiology and what behaviour
to cloak the fossil bones.

Later in tliis chapter, we shall discuss diosc trends for which
no adaptive meaning has "as yet been discovered. But it seems
clear that the considerable majority are definitely adaptive. So
obvious is this conclusion that it has found expression in the
current phrase adaptive radiation (first employed as a generaliza-
tion by H. F. Osborn; see c.g. Osborn, 1910). This is employed
to cover the well-known fact that large systematic groups usually
contain representatives adapted to a number of mutually exclusive
ways of hfe. The converse principle is that of the parallel physio^
logical or structural adaptation shown by the most diverse kinds
of animals confined to a single type of habitat (pp. 430 ff; and
examples in Hesse, Alice, and Schmidt, 1937). Adaptive radia-

EVOLUTIONARY TRENDS

487

tion is most obvious in the case of classes and sub-classes, but
may be traced both in higher and lower systematic units: how-
ever, in phyla and other units of high rank, the phenomenon is
manifested only on very broad lines, while in small groups such
as faniihes the type is in general so much restricted that the
radiation is neither so many-sided nor so obvious.

Thus classes and sub-classes provide the optimum size of group
in which the phenomenon may be studied: and in such cases,
whenever paleontological ewdence is available (as it is notably
in the placental mammals, but also in the reptiles and other
groups) the adaptive radiation is seen to be the result of a number
of gradual evolutionary trends, each tending to greater specializa-
tion— in other words to greater adaptive efficiency in various
mechanisms subservient to some particular mode of Hfe. As we
have already pointed out, adaptive radiation is ecological diver-
gence in the grand manner. It is the large-scale group manifes-
tation of the process whose details in minor systematics we have
discussed under the head of ecological speciation; and each
single adaptive trend also shows the phenomenon of successional
speciation.

In typical cases of adaptive radiation, a number of fines take
their origin in a generalized early group. There has been some
dispute among paleontologists as to the degree of generalization
to be expected in an ancestral form (see Gregory, 1936). For
instance, Henry Fairfield Osborn and his school wished to
extend considerably the principle of parallelism in (mammalian)
evolution, by assuming that in each group numerous separate
lines of descent run parallel far back into geological time, before
divergence from a common ancestor can be postulated (even
for the orders of modem placental mammals, common ancestry
is, by authors of this way of thinking, frequently assumed to date
back to the Upper Cretaceous); in correlation with this view,
the Osbom school further assumes that “even any remote
ancestors of any type must, in order to be admitted as such,
already exhibit’ unmistakable signs of the characters which are
very evident in their descendants”. Thus Miller and Gidley deny
to the Eocene rodent Paramys any ancestral significance for

488 evolution: THE MODERN SYNTHESIS

modem rodents such as sqiurrels and beavers, became it exhibits
no trace of the speciaHzation which these modem forms po^ess.

On the other hand, most paleontologists do not shrink from
the idea of radical transformation and apparent new ongm ot
characters within a line. Thus W. D. Matthew regards Paramys
as ancestral at least to the squirrels and beavers. There wo d
appear on general grounds no reason to accept the views of the
Osborn school. At some time the specialized mmt cemmly have
arisen from the generalized. However, jmt beea^ the ancestt^
type is so generalized, it is often, in view of the imperfecttons
of foe fossil record, very difficult to pmh foe history of a given
line back beyond the point at which foe first obviom signs of
its characteristic specialization appear. The stock at this stage ot
its evolution is often a variable one, and may show numerom
combinations of charaaers not found in any of the later types
derived from it. Specialization often consists partly m foe restric-
tion of the character-combinations found; and for the rest,
chiefly in quantitative alterations in the relative development of
this or that character. The process of specialization m all fines
continues steadily, but with different intensity in different Imes,
for a considerable time, which in the higher mammals at lemt
seems to last for between ten and forty million years; eventually
change ceases, and the specialized type either rapidly becoines
extinct or else continues unchanged for further geological perio s.

A further feature of such trends as have abundant fossil
documentation, such as foat of the horses, is foe amount o
parallel evolution foat occurs. Closely related stocks appear to
develop along similar lines, although frequently one line will
show acceleration in one adaptive trend, such as the specializa-
tion of foe grinding mechanism of foe teeth, with relatively
slow development in another, such as foe specialization of the
hoofed foot (see Matthew, 1926, and Stirton, 1940, for Imrses;
Osbom, 1929 and 1936, for titanofoeres and for elephants;
Swinnerton, 1921, for various invertebrates; and pp. 5 t 4 scq.).

Another feature of trends that are well-documented by fossils
is foe great -amount of variability that often occurs at any one
time, with consequent marked overlap at different levels. Thus

EVOLUTIONARY TRENDS 489

Trueman (1922) investigated the evolution of a curved Cryphaea
type of shell from a flat Ostrea type. The curvature in the flattest
shells from the lowest level investigated was only 10°, while in
the most curved shells from the highest level it was 540°.*
But the range of variation at five successive levels was as follows :

degrees

No. I 10-130
No. 2 100-340
No. 3 180-400
No. 4 220-500
No. 5 270-540

In general, no sharp line can be drawn between long-range
trends extending over scores of millions of years and short-range
trends of tmder a million years (see Swinnerton, 1932).

Swinnerton (1940) has investigated the same evolutionary
trend in more detail in another Ostrea-Gryphaea lineage. He finds
the same great range of variabihty at any one time. He has
further been able to prove, by interesting graphic methods, that
in certain characters the later communities differ from die earlier
merely in a restriction of the original variabihty, whereas in
other characters they have moved pardy or wholly beyond the
Umits found in the original community.

We will for the moment leave out of consideration those
advances (though they too are adaptive) which concern higher
all-round organic efficiency rather than greater efficiency in
relation to a particular environment or mode of life, and which
are better classified under the head of biological progress dian
under that of specialization: these will be discussed in Chap. 10.

The process of adaptive radiation may be illustrated by die
group of placental mammals. From the small and generalized
terrestrial forms of the end of the Cretaceous and the very
beginning of the Cenozoic, lines radiated out to take possession
of different environments.. Two quite separate lines became fully
aquatic, one of flesh-eaters culminating in the whales and por-

* Measurements expressed in degrees of total coiling instead of in terms
of the spiral angle, as given by Trueman.

490 evolution: THE MODERN SYNTHESIS

poises, the other of herbivores leading to the ^ sea-cows and
manatees. Still another line, that of the seals and the sea-hons,
branched off from the carnivore stock and became aquatic except
for reproduction. The bats meanwhile speciaUzed on aenal lite,
and the primates on Ufe in trees. The main ground-hvmg forms
belong to five chief branches— the carmvores, the rodents, the
elephants, the odd-toed and the even-toed ungulates. The rodents
specialized for gnawing, the carnivores for the capture of large
living prey; both ungulate groups, though quite separacte m
evolutionary origin, became highly adapted to a herbivorous
diet of grass or leaves and, in the most advanced types, to rapid
locomotion; tlie elepliants concentrated on a different type of
vegetarian specialization, with the aid of tusks, trunk^ and large

bulk. . ,

Among other groups, the South American edentates or

Xenarthra are instructive. They represent the surviving remnante
of a primitive early mammalian stock, and are not charaetenred,
as are the successful groups, by one predominant specialization.
On the contrary, their affinities are revealed only by compara-
tive anatomy, and they show remarkable divergent .specializa-
tions — the armadillos to protection by heavy armour, the ant-
eaters to an ant and termite diet, the sloths to an upside-down
arboreal existence, and the recently extinct ground-sloths to a
sluggish herbivorous life coupled with great bulk. It appears
that they have only been able to survive through embarking on
a secondary adaptive radiation of their own, superposing high
ecological specialization on a primitive organizational ground-

plan * . , . .

Something of the same sort has occurred with the inscctivores
—we need only think of mole and hedgehog— though the
members of this group have in large measure survived by
remaining generalized and of small size and by occupying humble

niches in the economy of life. _

Other groups, however, have disappeared entirely, notably
the higher creodonts among the cariiivores, and among vegeta-*

* A similar secondary radiation, but here correlated with more complete
competition, is seen in marsupials in the Australian region: see pp. 49i-

fiVOI-UTlONARY TUliNDS 491

riaiis, the amblypods, the titanothcrcs, the typothcres and their
relatives, the chalicotheres and the haluchitheres. All these were
specialized, and many of them of large bulk. In every case it
appears that they were extinguished because with their primi-
tive general organization, notably as regards the size and
efficiency of their brain, they were unable to compete success-
fully with the later-evolving carnivorous and herbivorous
lines.

Each successful line of course radiates further into sub-lines.
Among bats, for instance, there arc fmit-eaters, insect-eaters,
fish-caters, and blood-suckers. Among cetaceans there arc the
giant whalebone food-strainers, the big toothed whales special-
ized to feed on deep-sea cuttlefish, the carnivorous killer-whales
attacking other marine mammals, and the porpoises and dolphins
specialized for fish-cating. Even among seals there is marked
adaptive- radiation, some eating fish, others ccphalopods, otlicrs
crabs, and still others penguins. Still finer adaptive specialization
takes place witltin the sub-lines. Emerson (1938) gives a valuable
summary of the adaptive radiation of termites, which is largely
concerned with the type of nest-construction. We have given
examples from birds in Cliapter 6 (p. 325).

It is instructive to compare the adaptive radiation achieved
by different groups. The marsupials, for instance, that were
isolated in the Australian region underwent adaptive radiation
quite separately from other mammals clsewhcic, whether marsu-
pials or placcntals. The fact that they alone among marsupials
were able to specialize to this extent is doubtlc,ss a large-scale
example of the phenomenon noted in Chapter 6 (p. 324), of
the greater degree of differentiation made possible by reduced
competition from other types. However, the number of s].x:ciali-
zations achieved, and their efficiency, was not so high as in the
placcntals. Tliis in all probability is to be ascribed to the lesser
scope for variation and the lesser degree of selective pressure;
this is due to the smaller size and less varied nature of the area,
which in their turn restrict the total numbers of organisms in a
species, and therefore the potential of variation, and also limit
the numbers of different ecological niches. Some of the special-

492 evolution: THE MODERN SYNTHESIS

ized trends are extremely similar to those found in placentals.
For instance, the marsupial mole and wolf show a remarkable
parallelism with their placental counterparts. In many cases,
however, the same general type of specialization is achieved,
but in a different way. The kangaroos are the outstanding
example of this. They are well adapted to Hfe on grassy plains;
but nothing similar to them in detail has been evolved among
placentals as dominant plains herbivore, and among marsupials
nothing has been evolved similar to the placentals’ main speciali-
zations for plains Hfe — ^the horses on the one hand and the
antelopes on the other.

Some lines are altogether lacking in the marsupial radiation:
c.g. neither aquatic nor fully aerial forms were evolved. Others,
such as carnivorous types, are relatively poorly de -eloped; but
still others, such as small arboreal types, are more extensively
developed than in placentals. In general, however, . adaptive
radiation saw to it that the main ecological niches are occupied
by the Australian marsupials, even though the methods of occupy-
mg them frequently differs from those adopted by the placentals
in their radiation. There is little evidence that intrinsic variability
or other inherent properties of the stock have much to do with
the differences between the two sub-classes.

The evolution of the Australian marsupials demonstrates
adaptive radiation on the part of a medium-sized taxonomic
group restricted to a medium-sized area. Adaptive radiation may
be seen in much smaller areas as well as in much smaller groups.
Thus modem work (Yonge, 1938(1, 1938/j) indicates that the
remarkable prosobranch molluscan fauna of Lake Tanganyika,
which is unique in fresh waters both in abundance of species
and in special tjrpes, is not (as was originally suggested) derived
from a part of a Jurassic marine fauna cut off in the lake, but has
evolved in situ from forms already adapted to fresh water. All
fresh-water gastropods are herbivorous: this is proved for all
the Tanganyika forms by their possession of a crystalline style.
They have radiated into a variety of forms, adapted for living
at different depths and in waters containing different amounts
of sediment, and for securing their food in radically different

KVOLUTIONARY TRENDS

493

ways. However, it is interesting to note that the radiation is
limited in one important particular— no carnivorous types have
been evolved.

Similar local radiation permitted by long isolation Itas been
shovm to occur in the gammarids and other forms of Lake Baikal,
the cichlid fish of Tanganyika and other African lakes (p. 334),
and certain birds of oceanic islands (p. 325). As we should expect,
all degrees in amount of radiation appear to exist.

The adaptive radiation (or rather radiations, since several were
superposed) of the reptiles during the Mesozoic Period is per-
haps more comparable with that of placental mammals than is
tliat of the Australian marsupials, since they affected a major
group in the main land area of the globe. In this case all possible
main lines were evolved, including the full aerial and a dispro-
portionately large number of aquatic types. The excess of lines
tending towards very great bulk is also prominent. This fact
looms over-large in most discussions of die subject, and it is
often forgotten, even by professional biologists, that small types
adapted to erect ^ well as to quadrupedal running, to arboreal
life, etc., were also evolved. Here again there is no evidence of
any restriction of variability; the peculiarities of the reprilian
radiation, while in part due to the inherent properties of the
reptihan stock (scaly covering, small brain, etc.), appear to
depend in the main on peculiarities of the physical and biological
environment of the period.

The essence of adaptive radiation thus consists first in the
invasion of different regions of the aivironmcnt by different
lines within a group, and secondly in their exploitation of differ-
ent modes of life. In both cases progressive adaptation is at work.
In the first case this may lead to wholly new parts of the environ-
ment being colonized: for instance the sea and the air formed
no part of the environment of the original mammalian stock.
In the second case it may lead to wholly new organic arrange-
ments: for instance binocular and macular vision in higher
primates, the baleen filter of whalebone whales, or the ruminant
stomach in higher even-toed ungulates.

494

evolution: the modern synthesis

%. THE SELECTIVE DETERMINATION OF ADAPTIVE TRENDS

The trends seen in adaptive radiation would appear to present
no difficulties to the selectionist, .and it is hard to understand
why they have been adduced as proof of non-adaptivc and
intemally-dctcrmined orthogenesis. Whenever tliey are truly
functional and lead to improvement in the mechanical or neural
basis for some particular mode of life, they will confer advantage
on their possessors and will come under the influence of selection ;
and a moment’s reflection will show that such ^lectton will
continue to push the stock further and further along the hne

of development until a limit has been reached. , - i

This limit is usually determined by quite simple biomechanical
principles. A horse camiot reduce its digits below one per foot
nor can it, with a given body-size, increase the complexity ot
the grinding surface of its molars beyond a certain point without
making the grinding ridges too small for the food to be ground.
The selective advantages of mere size, wliich must often be
great in early stages of a trend, will be later offset by reduction
of speed, or difficulties of securing sufficiency of food, or, in the
final limit in land animals, by the relative increase of skeleton
necessitated.* There is a limit to the acuity of vision, to the
streamlining of aquatic form, to the length of a browser s neck,
wliich can be useful or indeed possible to hawk or vulture, to
whale or porpoise, to gerenuk or giraffe.

When tliose biomechanical limits have been reached, tlie trend
ceases, and the stock, if it is not extinguished through the increas-
ing competition of other stocks which have not yet reached the
limits of their trends, is merely held by selection to the point it
has reached. Ants, in some ways the most successful of invertc-

* If the same proportions are retained while absolute size is increased, cro^-
sectionai area of bone inareases as the square of linear dimensions, but weight
to be supported as their cube. After a certain limit the bone is unable to support
the weight. For instance, human thigh-bones will break if called on to support
about ten times the weight they now support, l^us a tenfold increase of man s
linear dimensions would bring him to the point where he could no longer
support his own weight, since cross-section of a thigh-bone would increase a
hundredfold, while weight would increase a thousandfold, and so each square
inch of femoral cross-section would be called upon to support ten times as much
weight. (See D*Arcy Thompson, 1917* Chap. 2; Haldane, 1927b, p. 18.)

EVOLUTIONARY TRENPS 495

brate groups, reached the hmits of specialization at latest by the
Oligoccnc, and have shown negligible evolutionary changes in
the succeeding 30,000,000 years (Wheeler, 1910). The titano-
theres reached theirs long before most placental trends had
been achieved, and accordingly were later extinguished by
the competition of more efEcient rivals. In general, the most
successful mammahan groups reached their limits in the
Pliocene.

One important fact must be stressed, since it is often over-
looked by those who would uphold an orthogenetic as against
a selectionist interpretation of such trends. It is that the environ-
ment to which a given line becomes adapted is organic as well
as inorganic; it includes all other forms of life with which the
type comes into ecological relation, as well as purely physical
and cHmatic features. Sometimes the inorganic environment
changes markedly, as when there is a climatic revolution, such
as occurred at the end of the Cretaceous; but in general it is the
organic environment which shows the more rapid and important
alterations.

Thus the evolution of the ungulates is not adapted merely to
greater efficiency in securing and digesting grass and leaves.
It did not take place in a biological vacuum, but in a world
inhabited, inter alia, by carnivores. Accordingly, a large part of
ungulate adaptation is relative to the fact of carnivorous enemies.
This applies to their speed, and, in the case of the ruminants,
to the elaborate arrangements for chewing the cud, permitting
the food to be bolted in haste and chewed at leisure in safety.
The relation between predator and prey in evolution is somewhat
like that between methods of attack and defence in the evolution
of war. In recent naval history, for instance, an advance in tfic
efficiency of big guns has immediately put an additional premium
upon advance in armour-plating, and vice versa. Sometimes
advance is so great that an entire method of attack or defence is
rendered obsolete. The improvement of artillery led to the
abandonment first of fortified castles and later of city fortifi-
cations as methods of defence: machine-guns and barbed wire
forced the abandonment of the cavalry charge as a method of

496 evolution: the modern synthesis

attack. Such radical changes have their biological parallels in the
entire or almost entire extinction of a group.

The dependence of adaptive trends on the organic environ-
ment is shown in a diagrammatic manner in the relation between
carnivores and herbivores; but more subtle dependence will exist
wherever two types are brought into ecological competition or
interrelation.

In addition, the organic environment of an individual includes
the rest of the species. This is a truism so obvious as often to be
forgotten; but since so much of selection depends on intraspecific
competition, it is of great importance. When ail ancestral horses
could run only moderately fast, an additional premium would
be placed on a little extra turn of speed; when the ancestral seal
had first taken to the water, a better streamlining and a more
eflScient flipper would give their possessors a definite advantage
over their fellows. When the biomechanical limit of speciah-
zation has been reached, such advances will no longer be possible,
and selection can only act either by keeping the species up to
the limit or by encouraging adaptive changes in other characters,
such as intelligence or reproductive efficiency. (Sec pp. 478 seq.).

Thus partly in relation to odier species, partly in competition,
with others of their own species, a constant selection-pressure
will be exerted, causing adaptive trends, once begun, to be
specialized towards a limit

That adaptive radiation is essentially a product of selection,
not the outcome of any intrinsic tendency, and is relative to
environmental conditions, is further shown by the fact that
when stocks are removed from competition or find themselves
in special environments, they may show renewed adaptive
radiation, although this has virtually ceased, at .least does not
take place, elsewhere. This is well exemplified by the gammarids
in Lake Baikal (Koromeff, 1905-12), by the fish in certain
African lakes (p. 324), and on a larger scale by such examples
as the Australian radiation of the marsupials just mentioned.
Similarly the enormous plasticity of e.g. pigeons under artificial
selection is proof that their previous stability was the effect of
selection-pressure, not of any reduction in intrinsic variabiUty.

EVOtUTIONARY TRENDS

■m

The same is clemonstrated by the development of flightless or
giant birds oir oceanic islands, where selection will act in a
new way. (See also p. 129.)

Theoretically, it may be possible to distinguish the problem
of the origin of adaptive trends from that of their maintmance,
once originated, at least in certain cases. For instance, the full
evolution of all the adaptations associated with the habit of
flatfish of lying on one side on the bottom, presents no particular
difficulties in the way of a selectionist interpretation, while tlic
first evolutionary step towards asymmetry is much harder to
envisage. Even here, however, we find half-way stages. The
primitive genus Psettodcs, for instance, has the originally lower
eye near the dorsal edge of the body, not on the secondarily
upper surface as in all other pleuroncctids. When we further
consider that various fish have the habit of occasionally lying
on one side on the bottom, the problem does not appear quite
so serious as at first sight. Thus the suggestion put forward by
Goldschmidt that tliis trend and others such as the asymmetry
of gastropod molluscs were initiated by abrupt mutations, though
afterwards maintained aird perfected by selection, becomes
improbable and redundant (pp. 456, 552).

3. THE APPARENT ORTHOGENESIS OF ADAPTIVE' TRENDS

The only feature inviting orthogcnctic explanation is die direc-
tive character of evolutionary trends, their apparent persistence
towards a predetermined goal.* But on reflection this too is
seen to be not only explicable but expected on a sclcctioilist
viewpoint. Over three-quarters of a century ago. Bates (1862)
pointed out this fact. He wrote, with reference to mimicry,
“The operation of selecting agents, gradually and steadily bring-
ing about the deceptive resemblance of a species to some other
definite object, produces the impression of there being some

* Berg in his book Notnoj^em'sh (1926) gives numerous interesting cx«imples
of trends which he puts down to orthogenesis. Many of these, however, are
covered by the consideration advanced in this section, while others would appear
to fall under the head of consequential evolution (§ 6). The wealth of examples
which he cites is worthy of detailed study from the selectionist viewpoint.

498 evolution: THE MODERN SYNTHESIS

innate principle in species wliicii causes an advance of organi-
zation in a special direction.” And lie later makes it clear that
he would apply the same reasoning to all other adaptive trends
{see also Poulton, 1931, who quotes Bates).

One difficulty that is often overlooked by believers in ortho-
genesis is the curious di&rence between related groups in regard
to the number of separate divergent trends to which they give
rise. Thus the horses are often considered as constituting but
a'smglfi trend, though as we have seen there are numerous minor
divergencies, and the extent of these has been stressed by Stirton’s
recent detailed studies (1940) as against Matthew’s classical
work (1926).

The evolution of this group can no longer be represented by
“a pine-tree with one main stem and insignificant side branches”
(H. E. Wood, 1941). But Wood goes on to point out, that even
so, its development is extremely simple when compared with
that of the closely-related group of the rhinoceroses. These show
a considerable number of highly divergent trends, including that
leading to the gigantic baluchitheres, and another to the semi-
aquatic amynodonts. Sometimes one and the same sub-group
shows a single-track trend in one geographical area, but complex
polyphyletic divergence in another. There is a considerable
amount of parallel evolution as well as divergence.

This is an extreme in the opposite direction from the horses.
Most groups of comparable taxonomic ranJc show an inter-
mediate degree of divergence.

It is impossible on orthogenetic principles to explain why
one group should contain in posse the tendency to show marked
divergent radiation and another comparable group should not,
why one should form Uvice or four times as many orthogenetic
trends as another. (Sec also Crepis, pp. 372 scq.).

To revert to the determination of single trends, it is clear
that, once a trend has begun, much greater changes will be
necessary to switch the stock over to some other mode of life
than to improve the arrangements for .the existing mode of life.
T. H. Morgan (1925, p. 148) has put this point very clearly.
“It has been pointed out that the power to reproduce itself puts

EVOLUTIONARY TRENDS 499

the problem of tlie construction of a living organism on a
different footing from the construction of a complex machine
out of inorganic (non-living) material. This question is so
important for the theory of evolution that its significance must
be further indicated.

“Whenever a variation in a new direction becomes estab-
lished, the chance of further advance in the same direction is
increased. An increase in the number of individuals possessing
a particular character has an influence on the future course of
evolution — ^not because the new type is more likely to mutate
^ain in the same direction, but because a mutation in the same
direction has a better chance of producing a further advance
since all individuals are now on a higher level than before.”
(Morgan might have added that for the same reason mutations
in other directions will have a worse chance than before.) “When,
for example, elephants had trunks less than a foot long, tfie
chance of getting trunks more than one foot long would be in
proportion to the length of the trunks akeady present and to
the number of individuals in which such a character might
appear. In other words, evolution once begun in a given direction
is in a favourable position to go on in the same direction rather
than in another, so long as the advance does not overstep the
Hmit where further change is advantageous.”

In the same way, too sudden an advance, eVen if mutations
for it were all available at 6ne time, would often be non-
advantageous or even disadvantageous. We can see diis from
analogies with human constructions. In the evolution of the
motor-car, the substitution of four for one or two cylinders
was a great improvement: it had “survival-value”. However,
not until the majority of cars came to be four-cylindered was
the additional advantage of still more cyliidcrs of sufficient
appeal to give the six- or eight-cyUndered engine any consider-
able advantage in the market. Agam, we can readily see drat the
isudden “development” of full modem armour-plate on the
earliest ironclads would have been actually disadvantageous, since
it would have reduced their speed relatively to less heavily pro-
tected sliips, without conferring any corresponding benefit in

500 evolution; THE modern SYNTHESIS

the way of defence gainst the comparatively inefficient pro^
iectiles of the day. Only when the range and piercmg^powcr
if the projectiles increased ffid increase of armour become

sudden large increase in size and power of a c^-
niwe wiffiout corresponding advance in its prey might , be
fettouTto the specie, since it might kill out or markedly
reduce its own food-supply. Again, a marked “

one character might be non-advant^eous m the absence of

corresponding improvements in correlated characters.

Spoint,\oJever, U cly of *eo„tical inters, for orga^c
rvobdoo, for the dmple reason Aat the supply of mutatt^s is
so slow, Ltd the mutations which can be med appeal to be of
such small extent, that really sudden and unadjusted advance is
impossible. Most adaptive spccialiaanon therefore cannot Help

sum up the position as follows. Smee sel^tion can
only build with the materials provided by mutation; smee
mutation is a slow process, and since the matend it provide,
to be useful for selection, must be m the natme of small hncks,
it follows that the chances are overwhelmingly m favour of the
small changes needed to confer advantage in preserratg the
existing trend, turning up (and therefore bemg acted upon)
before the larger changes needed to confer advantage m another
mode of life have had any likelihood of occurring. A speaahzed
line thus finds itself at the bottom of a groove cut for it by
selection; and the fiirther a trend towards speciahzation ^has
proceeded, the deeper will be the biological groove m which it
has thus entrenched itself. Thus specialization, in so far as it « a
product of natural selection, autoinatically protects itself agamst
the likelihood of any change save further change in the same

direction. ,

Plate (1913, p. 511), who reaches very similar conclusions,

proposes the term orthoselection for selection promoting the
tinuance of an adaptive trend. It is surprising that this useful
term has not come into more general use. That this apparent
orthogenesis is determined functionally and not by some inner

EVOLUTIONARY TRENDS 5OI

clockwork of the germ-plasm which predetermines a progressive
change in structure, is excellently shown by the evolution of the
elephants (see summary in Lull, 1917, p. 588). These began
their career by an elon gation of the muzzle, involving the enlarge-
ment of both jaws and both upper and lower incisor tusks.
Before the beginning of the pliocene, this process had reached
what appears to have been a mechanical limit. In the later
evolution of the stock, the jaws were shortened, the trunk
elongated, and the lower tusks aboHshed. The effective reach of
the animal for its food was continuously increased: but the
structural basis for this functional change was wholly altered,
the elongation of the trunk being substituted for that of the
jaw. It is impossible to stretch the principle of internal ortho-
genesis to cover a process of this type.

Another reversal of trend is that shown by the baboons (see
e.g. Gregory, 1936), in which a secondary lengthening of the
face into a muzzle has occurred subsequently to an original
trend in the opposite direction, as exemplified by most Old
World monkeys. This difiers from the case of the elephants in
that here the trend itself appears to have been reversed, while
in the elephants the functional trend continues, and only the
means for realizing it are altered.

While on the subject, we may deal with a cognate point,
DoUo’s so-called law of the irreversibiUty of evolution. This is
an empirical fact of paleontology, but would appear to be merely
the result of probability and what we may call biological con-
venience: That it involves no intrinsic necessity is shown by the
experimental findings of geneticists on polydactyly in guinea-
pigs. The tame guinea-pig, like other members of the genus
Cavia, normally possesses but four digits (ii-v) on the front
feet and three (u-iv) on the hind. By selec^ve breeding from
the individuals possessing one or both hind littb toes (digit v),
which appear sporadically in certain domestic strains, a stock
can be produced which always possesses this digit (see Pictet,
1933; Wright, I 934 l>). The basis for the character seems to be
an iteration in the digital embryonic field, permitting it to be
divided into more digital units than in the normal type (p. 55 °).

502 evolution: THE MODERN SYNTHESIS

The little toes thus experimentally resurrected appear perfectly
normal, so that man has been able to build up a stock which
was in full possession of a hind little toe that the wild species
and indeed the whole genus had definitely lost many millions
of years ago. Thus nature no more abhors reverse evolution than
she abhors a vacuum.*

Muller (1939), in a carefully reasoned review of the subject,
comes to the same general conclusions. He points out that not
only will the old characters resurrected by reversed evolution
never rest on a genetically identical basis, but that with complex
characters they will inevitably cease to be phenotypically identical
or even closely similar. See also Needham’s interesting discussion

(1938)-

The matter is complicated by the fact that the muscle and
tendon supply is to a considerable extent independent of the
bones and dermal structures Q. P. Scott, 1938), so that normal-
looking digits may be abnormal functionally. In some cases,
however, extra digits appear to be quite normal. This illustrates
the difficulty of restoring (or independently evolving) a character
depending on many distinct major factors.

Gregory (1936) also maintains the correctness of Dollo’s Law.
First he points out that DoHo himself asserted that the “Law”
applied only when an organ is wholly lost. Thus cases of per-
fectly definite “reversal of evolution” which happen only to
apply to changes in proportion are excluded by a quite arbitrary
definition. Then Gregory maintains that structures which are
regained are never entirely identical with the corresponding ones
that were originally lost. Thus it has been shown that in the
occasional three-toed horses that occur to-day, the extra digits
“do not have the same coincidence with the carpal bones as do
the side toes in the feet of the extinct Hipparion”. Sometimes he
goes further. Thus, though mastiffi and St. Bernards may show
the big toe which is lacking in wild Canidae, “it would be

* Wriglit (op. cit.) has furdier produced another stock which, in addition
to little toes, dmost invariably possesses a thumb (fore digit i). This stock, how-
ever, can only be maintained in the heterozygous state, since the genes concerned
are lethal when in double dose, although the thumbs produced appear quite
normal

EVOLUnONARY TRENDS 503

difEieult to prove that this so-called big toe is truly homologous
with the true first digit of primitive mammals”. Here the words
truly and homologous appear to beg several questions, to which
it is probably impossible to give an answer. If it is maintained
that a regained organ is never absolutely identical with that
which was lost, this is probably true in the great majority of
instances. However, not only do some of die cases of extra toes
in guinea-pigs seem to contradict this (though even here, certain
characters such as coat-colour, etc., are almost certain to liave
altered since the organ was first lost, thus rendering it at its
reappearance not completely identicsl), but wherever a “normal”
character has been markedly altered (as in fowls’ combs) or totally
suppressed (as in the horns of cattle) by a dominant mutation, it
is - obviously possible, provided certain genetic precautions are
observed, to re-obtain the identical normal character from the
heterozygous form after an indefinite number of generations.

Regan (1924) has given an example, not based on experi-
mental proof, of reversed evolution in fish. This concerns the
re-acquisition by the Loricariidae of denticle-like structures on
the scales. Another case is the return of later amphibians to the
series of simple neuromast pits in place of die roofed-in groove
constituting the lateral line of higher fish in early amphibia.
The pit stage is ontogeneticaUy and phylogenetically primitive.
This case may be accounted for by a quite simple alteration in
the time-relations of development.

DoUo’s Law should thus in the first place be restated in more
general form, and in the second place it should be regarded as
a mere rule and not erected into a principle. It is true that the
more complex an organ is, and the more completely it is lost,
the less likely it is to be regained in identical form, but this
depends on no absolute “principle of irreversibility” — only on
the high degree of improbabiHty of reversal in all of many
factors concerned.

Many trends which at first sight appear useless may turn
out on analysis to have fiinctional significance. For instance,
Malcolm Smith (1938) comments on the trend in aganiid H2ards
towards reduction of the structures of the middle ear. The

504 evolution: THE MODERN SYNTHESIS

functional reason here may be the risk of damage to these
dehcate structures by hving prey struggling in the mouth, as
in the parallel trend seen in snakes, but Smith is inclined to a
purely orthogenetic explanation.

The same principles seem to apply in general to small-scale
adaptations as to long-range adaptive trends, except that since
such adaptations frequently concern only one particular func-
tion and not the organism’s main way of life, it should be easier
for evolutionary direction to be changed, and for adaptation to
set off on a new tack. In the matter of coloration in birds, for
instance, there is a balance between the advantages to be derived
from concealment and those to be derived from conspicuousness.
The former will be higher for defenceless species and in open
environments, the latter will be higher in males when there is
polygamy (stimulative value of display characters) or when
there is much rivalry between males as in territorial species
(threat characters), and in both sexes in gregarious species (recog-
nition markings). A shght change in habitat preference or repro-
ductive behaviour will speedily tilt the balance in one direction
or the other (see Huxley, 1923b, 1938a, 19386; and p. 426).

4. NON-ADAPTIVE TRENDS AND ORTHOGENESIS

Besides the usual trends constituting the radiation of a group,
most of which, as we have seen, appear clearly to be towards
adaptive specialization, there are others for which no adaptive
significance has as yet been found. The most striking are, natur-
aUy, those for which wc have direct fossil evidence. Watson
(1926) enumerates a number of trends observed in the extmct
group of amphibia known as Labyrinthodonts. These include
the flattening and broadening of the head and forepart of the
body; the shortening of the skull, resulting in the hypoglossal
nerve passing out posteriorly to the cranium instead of through
the basiocdpital; the gradual downward extension of the fore-
part of the cranial cavity ; the progressive diminution in ossifi-
cation of the cranium, and the &al disappearance of certain
bones; the development of an otic process in the pterygoid; etc.

EVOLUTIONARY TRENDS

505

These trends occur in a parallel way in a number of quite separate
lines, and almost synchronously. They proceed on their course
in spite of radical changes in the animals* biology, such as that
from aquatic to terrestrial and back to secondarily aquatic life.
And Watson states categorically that he can see no adaptive
significance in any of them.

Certain trends have been assumed to be orthogcnetically
determhied since their end-products appear to be more or less
pathological. Nopesa (1923) has termed such evolutionary
simulations of diseased conditions anhostia. In certain cases, as
with the pachyostosis and osteosclerosis wliich occurs in various
marme vertebrates, and wliich simulates certain accompaniments
of leukaemia, this seems to be a temporary means of securing
better respiratory adaptation to an aquatic life (Nopesa, 1923)
during the period when otlier more satisfactory but more
elaborate adaptations arc being evolved. We thus tend to find it
in the early stages of secondary aquatic trends (the permian Meso-
saurus, the triassic nodiosaur Pachyplcura, the lower cretaceous
lacertihan Eidolosaurus, the lower cretaceous ophidian Pachyophis,
the eocene cetacean Zmglodon); only in the vegetable-fecduig
Sirenia is it a permanent feature, but even so it has been much
reduced in the later history of the group. In one case only, the
pliocene sirenian Felsinotherium, does it seem to have become so
excessive as to contribute to extinction. Thus, since it docs not
represent an irreversible trend which becomes accentuated until
it ends disastrously, there is no need to postulate an orthogenctic
determination for it.

Aquatic vertebrates show another arrhostic condition of bone,
namely a retardation of ossification, exclusively or mainly of
cartilage bone. Nopesa (1930) cites Stegocephalia, modern
Amphibia, Chelonia, Ichthyosauria, and (to a shght extent)
Plesiosauria as showing this tendency. He further points out that
it is very similar to the retardation of ossification produced by
hypothyroidism. He suggests that in some as yet unknown way,
aquatic life damps down thyroid activity; if this view is correct,
then the arrhostia is a consequential efiect of aquatic life, and need
not be regarded as die result of a special orthogenctic trend.

5o6 ■ evolution: the modern synthesis

Nopcsa (1923) also points out that in certain cases of very
large size in vertebrates, arrhostic conditions simulating symptoms
of hyperpituitarism (acromegaly) may arise. It would, however,
appear probable that this again is a purely consequential effect
of the large size, which itself depends largely or mainly on in-
creased pituitary functions. If, as appears likely, the large size
itself (e.g. of certain dinosaurs), during the favourable climatic
conditions when it was evolved, was itself of advantage, ortho-
genesis need not be postulated, and the condition is closely
parallel with that produced in the St. Bernard breed of dogs by
artificial selection (p. 71). This interpretation is strengthened by
the facts concerning the role of the pituitary in the evolution
of tlie giant birds of the family Aepyomithidae as determined on
endocranial casts (Edinger, 1940). Here the size of the pituitary
relative to that of the forebrain increases over fourfold as absolute
size is increased from MuUerornis to the gigantic Aepyornis
maxitnus. Fmahy H. E. Wood (1941) and Goldschinidt (1940)
draw attention to the fact that numerous vertebrates — forms of
rhindeerotids (amynodonts), cave-bears, a number of fishes, etc.
— ^have developed a facies very similar to that of achondroplasia,
notably the “bull-dog” type of face. Here the implications are
not so dear as with acromegalic arrhostia, but at least there is no
more necessity of adopting orthogenesis as an explanation than
in any of the other cases cited. Arrhostia seems thus to be a
consequential rather than an orthogenetic phenomenon (pp. 525
seq.). See also Stockard (1938) on “pituitary” characters.

A succinct account of other noteworthy examples, in some
of which the non-adaptive nature of the change seems clearer,
is given by Haldane (i932<j, p. 23) and may be quoted here:

“Further observation of these marine races showing slow con-
tinuous evolution displayed an extraordinary group of phenomena
which are not obviously explicable on any theory of evolution what-
ever. Characters appear to go on developing past their point of
maximum utiHty. Thus the coiling of the Gryphaea shells [lamelli-
branch molluscs] went on until it must have been very difficult for
them to open at all, and impossible to open widely. This state of
affairs occurred several times, and always portended the extinction
of the race. The same thing sometimes happened in land animals.

BVOLUTIONAHY TRENDS 507

Thus in the ritaaiotheria [large oligoceiie hoofed mamtiialsl gigantic
size and horn development were the prelude to extinction in a num-
ber of separate lines' of descent. One is left with the impression that
the evolutionary process somehow acquired a momentum whidi
took it past the point at which it would have ceased on a basis of
utility.

‘'But sometimes another process occurred, which has been par-
ticularly studied in the Ammonites. These animals, which in a general
way resembled cuttlefish, made spiral shells with many chambers,
but only lived in the last of them, the others being presumably filled
with water or gas. The inner chambers were made by die young
animals, the latter by the adults. So wc can contrast the shell-making
activity of the same animal at different ages. We then find that the
earlier chambers often resemble those produced by the adults of ances-
tral forms some millions of years earlier. The phenomenon can be
especially well studied in the suture lines between different chambers.
The correspondence is not exact, and often new features appear in
the earlier stages which were not present in any ancestors. . . .
This is quite analogous to the phenomenon of partial recapitulation
seen in the early development of such forms as man. An early human
embryo has rudimentary gill-slits and a tail. Later, on it develops a
thick coat of hair which is shed before birth. Of course the gill-slits
and tail arc unlike those of any adult animal, and it has special organs
sucli as the umbilical cord which are not and never were found in
adults. But many of its features recapitulate those of its adult ancestors.

“All this can be explained on Darwinian lines. The less a new
adult character interferes with normal development the more likely
it is to be a success. When, however, it has been fixed in the adult
stage the complicated developmental process may well be slowly
modified so tliat the advantages of the new character appear ^earlier
and earlier in the life-cycle and its appearance is less and less abrupt.
This process is, however, likely to be very slow.

“So far so good, but in the later stage of Ammonite history a muclr
more surprising phenomenon occurred. A number of different lineages
began to alter in the opposite direction. Features appeared which had
not been seen for a hundred million years, but which strongly re-
sembled those of the earliest known Ammonites. The sutiirc-linc
became simplified, and the shell uncoiled. Sometimes the primitive
features seem to have been present right through the animafs life-
liistory. In otlier lines of descent (e.g. BacuUtes) the shell was at first
coiled, but in the fully adult animal it was straightened out. This
reversion to primitive type was always the prelude to extinction.
It happened on a large scale in the late Trias, when most of the great
Ammonite group^s died out. Then there was a brilliant renaissance

BVOL0TION:' THB 'MOBEIH S YNTHESIS

Joring the Liatjie perioJ, one of the older groops gmng me to Mny
„™ types. Bn. m epoch of archeism set m on^

C«.cSS, at Ae end of A« pmod A. las. Ammomtt d
The closing stages of Ammonite evolution were marked, not only
by retrogressionfbut by the appearance of new shcU types, with h^rpiii
binds’ Jin Hamites, or an asymmetrical snail-Iike spire as in Tmttebtes
These bizarre forms, however, were only temporarily successful. Atter

about 400 miUion years oflife the Ammomtes became extinct.

“The account here given is that due to Hyatt and Wurtemberger,
and is, I think, accepted by most paleontologists. However, Spa^ s
(1024) views on Ammonite lineages, which are easier to reconede
with Darwinism, command much support. I am not rompetent to
judge between them, but wish to state the anti-Darwinian position
as fairly as possible/*

Among other examples often adduced by paleontologists as
useless or eventuaUy harmful trends are those towards spmmess,
and over-ornamentation in trilobites, and towards excessive
development of the arm-skeleton in brachiopods. ...

The second case which Haldane mentions, that of me timo-
theres, can be more simply accounted for on Darwinian lines
(p. 534 )- Indeed Haldane himself (op. cit., p. 141) later argues
fbai- the development of apparently unfavourable characters «
a prelude to the extinction of a stock can in many cases (notably
unwieldy size and exaggeratedly large horns), be put down to
the biologically evil effects of intraspecific selection (p. 484)-
On die other hand, no selectionist interpretation of the over-
coihng of the Gryphaea lines, or of the secondary simplification
of ammonites, or of other bizarre preludes to extinction in other
groups, has as yet been given. A caveat should here be entered.
No living ammonites arc known. The compheation of suture
lines of the earlier ammonites and their simplification in later
forms have at least a simple mechanistic basis in terms of rate-
genes (p. 530). About their functional meaning we know next
to nothing, so that it is dangerous to maintain' that they were
in no sense adaptive. This is the view to which Bather (1920)
eventually came, after wide paleontological experience. It ma^j.
even be that we arc betraying our ignorance by not being able
to perceive the direct functional utility of the latest strangely
bent and partially involved types of shell. This, however, is

EVOLUTIONARY TRENDS 509

unlikely, and with the Gryphaea stiH less likely. An additional
comphcation is introduced by the fact that, as briefly noted by
Haldane, Cryphaen is not a true genus, but merely a name for
the final stage in the coiling of an ostreid shell, and that this
stage has been reached by several lines, starting their evolution
at different times (Arkell, 1933; p. 409). The fact tliat on
several occasions certain oysters remained flat while otlicrs pro-
ceeded to show tliis tendency to over-coiling which was even-
tually to lead to their extinction, is difficult to account for on
any hypothesis so far put forward, orthogenetic or otherwise.

It is, of course, possible tliat these trends, in themselves useless,
are correlated with adaptive trends in other characters (pp. 63, 206).
However, we must provisionally face an explanation in terms
of orthogenesis— i.e. of evolution predetermined to proceed
within certain narrow limits, irrespective of selective disadvan-
tage except where this leads to total extinction. It should be
noted that, even if the existence of orthogenesis in tliis cause be
confirmed, it appeare to be a rare and exceptional phenomenon,
and tliat we have no inkling of any mechanism by which it may
be brought about. It is a description, not an explanation. Indeed
its existence runs counter to fundamental selectionist principles

(p. 123).

Of course, if mutation-rate were high enough to overbalance
counter-selection, it would provide an orthogenetic mechanism
of a kind. However, as Fisher and others have shown, mutation-
rates of this intensity do not exist, or at least must be very rare.
Secondly, even if they did exist, they would not by themselves
provide an explanation of the real problem at issue, which is the
long continuance of apparently orthogenetic trends. For this, wc
should have to postulate not merely a high mutation-rate, but a re-
striction of the direction of mutation, so that new mutations with
high frequency would always be arising to produce further effects
of the same type. And of tliis there is no evidence whatever.*

* The work of folios (1930), claiming that induced mutations tend to occur
in a scries of successive steps with progressively greater cl?ect» appeared to
indicate a mechanism of this general type for mutations of low frequency. Later
researches, however (e.g. Plough and Ives, I934)» entirely failed to confirm
the existence of such “directional” or “progressive” mutation.

510 evolution: the modern synthesis

Orthogenesis of this sort, playing the major part in guiding
evolutionary change, with selection in a purely limitmg^ ^d
subsidiary role, may be called dominant or primary. While
dominant orthogenesis, if it exists at all, is rare and exceptional,
what we may call subsidiary or secondary orthogenesis is com-
mon enough. Under the head of subsidiary orthogenesis I mclude
phenomena which in the first place are of an orthogenetic nature
in that they Hmit the freedom of variation and therefore or
evolutionary change, and in the second pkee are subsidiary^ m
that they merely provide limits within which natural selection
stiU pkys the main guiding and shaping role. ^ ^

The first phenomenon of subsidiary orthogenesis with vvhich
we must deal is parallel variation (pp. 99. 211 , 395._43 i)- This is a
comprehensive term winch includes several distinct procesKS.
In the first place there is homologous mutation — the alteration
of homologous genes in the same kind of way. In the second
place there is parallel character-change— a similar phenotypic
effect,- produced, however, by mutations in different genes.
Parallel character-change may further be cither (a) homologous
or (b) purely superficial. When homologous, the same type of
developmental process is usually affected in the same kind of
way: when superficial, the phenotypic effect is similar, but is
produced by different developmental processes. This important
distinction between the different modes of parallel variation has
not always been clearly envisaged. (We must also remember
that truly homologous mutations may sometimes exert quite
different phenotypic effects in different species.)

We will take examples of the different categories. In certain
species o£ Drosophila, such as D. melanogaster and D. sitnulans, the
possibility of obtaining offspring from interspecific crosses has
enabled geneticists to prove that certain pardUel variations, e.g.
white eye and one yellow body variant, are due to truly homo-
logous mutation — ^i.e. the same type of mutation has occurred
in corresponding loci, or in other words in descendants of the
same ancestral gene (see Morgan, Bridges, and Sturtcvaiit, 1925 )*
The proof consists in the offspring of two similar mutants show-
ing the same mutant character, and not presenting a reversion

EVOLUTIONARY TRENDS ^511

to wild type, as would ocxur if die genes were not homologous.
In other cases, though this complete proof cannot be given
owing to sterility, the location of the genes in corresponding
sections of apparently homologous chromosomes is strong pre-
sumption of true homologous mutation. The blood-group genes in
man and apes are possibly another example. Interspecific grafting
(Stubbe and Vogt, 1940(1) may also demonstrate gene-homology.

In other cases the evidence is less cogent. Haldane (ipayd),
for instance, has collected the data on the colour mutations of
domestic and wild rodents and has shown that much parallel
variation has occurred in the various species and genera. To
deduce, however, as he is inclined to do, that these are ail due
to true homologous mutation, though in several cases probable,
is not logically justified; similar phenotypic effects are often
produced by mutations in non-homologous genes. There are, for
instance, several non-homologous mutations for pink eyes and
two for yellow body-colour in Drosophila (Morgan, Bridges, and
Sturtevant, 1925), and several for red eye-colour in Gammarus
chevreuxi (Sexton and Clark, I936i>). In rabbits, three separate mutant
genes may produce the “rex” coat-character (found also in otlicr
rodents) : Castle and Nachtsheim (1933). Thus parallel variation
may be due to non-homologous mutations producing parallel
character-change.

Another similar line of approach is provided by the data on
mutation-frequency in the laboratory, and on the proportion
of wild-caught individuals carrying mutant genes in a hetero-
2ygous condition. From these it has been estabhshed that various
mutational effects recur regularly in all organisms which have
been thoroughly investigated (see p. 396). Here again the effects
may sometimes be due to homologous mutation, sometimes
merely to homologous character-effect. In cither case, the presump-
tion is strong that a number of corresponding mutational effects
will recur independently in various related species of a group.

It is important to note that when non-homologous mutations
affect the same developmental process in the same kind of way
in different species, the resultant character-change may legiti-
mately be called homologous, even though neither the final

^^512 ■ ' evolution: . the mudein synthesis

character, nor the steps of the developmental process by which
it is generated, are precisely identical. In such cases the nature
of the developmental process provides a certain limitation or
canalization of the types of variation possible. Thus if melanm
pigment is present, a reduction in the intensity of its production,
however brought about, will result in dilution; a certam type
of chemical alteration of the process will result in htovm or
yellow mstead of black, and so on. When, as in many rodents,
the agouti pattern is normally present, with a yellow section on
a black hair, the chief modifications possible (apart from the
dilution of black or its alteration to brown, and the total inhi-
bition of all pigmentation) appear to be (i) either the extension
of yellow to cover the whole hair or its reduction to leave the
hair wholly black, or (ii) the presence or absence of a larger or
smaller area of yellowish-white on the belly.

In a similar way, the process of wing-development in Droso-
phila is such that numerous non-homologous mutations can
produce greater or lesser notching or truncation at the tip, and
various others can reduce the width of wing (see list of genes
in Morgan, Bridges, and Sturtevant, 1925)- In general, the siim-
larity of homologous character-changes is due to their innuenang
an identical developmental process in a similar way. Truncate
wings provide an interesting special case. Here Altenburg and
Muller (1920) showed that the chief gene for truncation in
some fashion “sensitized” certain developmental processes in
such a way that many quite independent mutations shortened
wing-length much more in the presence of this gene than in
its absence. Similar cases are now known for other processes
in Drosophila and for other organisms, and will become frequent
as work in physiological genetics progresses. In one sense, this
is a case of parallel variation, in another, of consequential evolu-
tion (§§6,7)* ...

Superfieijd parallel character-change is known specially tor

* The fact that the processes of development restrict the possibilities of ^ar'a-
tion has a further consequential effect, in the frequent existence of what Ooia-

schmidt (c.g. 1940 , passim) has called modifications that are piieno-

typically indistinguishabk from mutational effects. This phenomenon m its turn
may provide the basis for proresst's of organic selection (pp. 5^4)-

mimetic insects. Here selection-pressure has been dominant, and
has moulded dissimilar processes to give similar cfifects. A good
example is Papilio luctor and its mimic the romulus form of
P. polytes, in which the red pigrnents are chemically distinct, the
one turning yellow, the other purplish on appHcation of acid
(Ford, 1937). An even better case is that of a skipper butterfly,
Abantis levebu, which mimics Pierincs (whites) : the white pig-
ment of the skipper is a flavone, while that of the whites is a
pterin (unpublished information from Mr. E. B. Ford).

A curious non-mimetic case is that of Satyrus anthe, in which
two areas of apparently similar white are due to wholly different
pigments, in one a pterin derived from metabolic breakdown,
in the other a flavone derived by building up from a product of
the food-plant.

Parallel variation may tlius affect homologous genes and
homologous processes: or non-homologous genes but homo-
logous processes; or genes and processes both of which arc
non-homologous. The first two types, since they arc often
indistinguishable in practice, may conveniendy be lumped
together under the head of parallel mutation.

Finally, we must remember that owing to the alterability of
gene-expression by the residual gene-complex (pp. 64, 87),
even truly homologous mutations need not produce similar
phenotypic e&cts.

These facts have interesting evolutionary bearings. In the first
place, the existence of true homologous mutation shows us that
the classical post-Darwinian concept of homology cannot be
apphed to species. That concept equated correspondence in plan
of organization with common descent. This conclusion really
involves two steps, one a generalization of observation, the other
a historical deduction. The arm of a man, the wing of a bird,
and the flipper of a whale can be shown to be built on a common
plan: and it is deduced that the reason they arc all built on a
common plan is because their three possessors arc all descended
from a common ancestor (see pp. 391 scq.).

This explanation of the fact of homology by common ancestry
undoubtedly holds good for complex structures for whose

EVOLUTIONARY TRENDS

514 evolution; the modern synthesis

evolution a very large number of ^ steps were

biologist would venture to suggest that the pentadactylc hmbs

of vertebrates or the mouth-parts of insects could ^

separately evolved in more than one stock. But Ae fact of horno-

logous mutation shows that it need not hold
involving only one or a few mutations. Both Drosophila melano-
gaster md D. simulans have red eyes, but both
white-eyed mutants. The white-eyed types axe clearly homo-
logous in that they are due to corr^spondmg alterations m corre-
sponding parts of the hereatary constitution; but they canno
£ traced back to a common white-eyed ancestral secies. T^e
same is true of commonly recurring aberraPons withm a wild
genus or species-e.g. wHte-flowered bluetelk or genttans,
which may even establish themselves as small locah^oups in
nature. There is no ancestral white variety from which all me

white-flowered specimens are descended. .

More to our present purpose is the bearing of recurrent md
parallel variations on the phenomena of convergence and paraUe
evolution. In this case botii homologous mutation and homo-
logous char{u:ter-change will clearly be relevant.

We have mentioned (p. 488) the findmg of paleontologists,
that in fossil Bneages with abundant documentation, numerous
separate lines appear to pursue the same general teend, although
the rate of change of separate characters may differ; e.g. m the
horses, some lines, although often spedficafly or even genencally
distinrt, will lag behind the mean for the period as regards
complication and size of molars, while showing advmce beyond
the mean for reduction of digits; others will show the convene;
and still others will be at the .mean for both characters. The

same is true of the sea-urdiin Aherasfer (p. 32).

This can readily be explained if we assume that mutations
with similar effects are likely to turn up in related lines. It must
be observed, however, that selection-pressure is also necessa^.
ActuaUy, it will be the dominant factor, since it alone prescribes

the general direction of specialization.

The fact, noted above (p. 509), that the coiled Gryphaea type
of shell was independently evolved in oysters at several different

EVOLUTIONARY TRENDS 515

times and places may petlmps imply that it was of selective value
under certain temporary and local conditions (e.g. of greater
sedimentation). However, its apparent final harmfulness to the
lineages in which it occurs appears to rule out any simple selec-
tionist interpretation. As with so many paleontological riddles,
we may never learn the answer.*

According to Osborn (1936) and le Gros Clark (1934),
parallel evolution has played a considerable part in the geo-
logical history of the Proboscidea and the Primates respectively ;
and Brough (1936) gives interesting examples from bony fish.
Parallel evolution in the titanotheres, and its probable explana-
tion, is discussed on p. 534. Parallel evolution appears to have
taken place in several separated lineages of Jurassic hexacorals,
in each case tending to greater compactness of the corallites
(W. D. Lang, 1938).

Various mimetic resemblances, especially in synaposcmatic
“rings” of related species dl sharing the Mullerian advantages
of a common warning pattern, but also in some Batesiah cases,
will doubtless prove to depend largely on parallel mutations.
On the other hand, many mimetic resemblances are demon-
strably due to completely non-homologous character-changes
(see Carpenter and Ford, 1933, p. 31), so that selection must be
the essential agency in their production.

The sathe reasoning appHes to the interesting case cited by
le Gros Clark (1934, pp. 81-83) of the evolution of teeth in
Primates. The two sub-families of fossil Lemuroidea, the Nolh-
arctinae and the Adapmac, both show the evolution of a quadri-
tubercular from a trituberqular type of molar tooth. But whereas
in the Adapinae this condition is brought about in the normal
way by the development of a true hypocone as a wholly new
cusp from the cingulum, in the Notharctinae a pseudo-hypoconc
is formed by the fission of the original protoconc into two cusps.,

* One of the features characterizing the evolution of the Grypha^a from the
Ostrea type is a progressive increase in absolute size. Swiunerton studied

the evolution of a Imeage which rather more than doubled its linear dimeiiisioxis.
during part of the Lower Liassic, He estimates that this change proceeded at
the i^te of an increase of i per cent in size in about i,ooo generations (cf. p, 6i n.
on the rate of evolution in horses). Thus the effect may be consequential (p. 535)

5i6 evolution; the modern synthesis

The fact that the trend towards the quadritubercuiar condition
cannot be due to paraUel mutation constitutes additional evidence
in favour of a selectionist interpretation. ^

In groups showing polymorphism, the same varimt tyFS
n^^ m^^over and bver again in different species. A classical
caJ is the existence of apparently identical variant phases m
Sding and ground-colour m the snails Cepaca hortcus,s .ud C.
nmorafis (p. »)■ Recently Rubtzov (i 935 ) has shown that com-
plex rolour-pattcrns recur as normal variants not only m related
Lcies but Jelated genera of grasshoppers. Thus ot six colour-
phases, aU recur in five species of Chorthippus, and four or five
hi six others, while two to five also recur in various species o
seven other genera. Within genera, parallel evolution may often

o^r sometimes to the confusion ofthesystematist who a^mpts

a phylogenetic classification. Thus in die Austrahan bird genus
aImI (Mayr and Serventy. 1938) a brighter-coloured ru^,
a marked Vatlm on the head, strea^g on die breast ^d
lengthening of the taU have aU occurred more than once. In the
butirfly genus Colks, Mr. E. B. Ford informs me. numerous
species have yeUow males and dimorphic females, yellow or
white (cf. p. 98). In some cases this is due to a special genetic
mechanism (p. 99). In others, selection acting on very similar
germplasms with very similar capacities for
produce such a result; this would m gener^ apply to tlfc mime
L parallel regularities afforded by the Geographical Rules
Tf systematJ (see p. zn), though here non-homologous

variation doubtless plays a larger role.

In general, it may be said that the fact of parallel mu ation
makes parallel evolution and certain types of convergaice likely
to occur, but only in such cases where parallel mutation is supple-
mented by parallel selection, or by special genetic mechanisms.

5. THE RESTRICTION OF VARIATION

Then we have restrictions on the amopnt of variation possible.
There is, for example, a great contrast between the uniformity
of snipe or most ducks as against the tendency of many species

EVOLUTIONARY TRENDS

517

of passerine birds to break up into gec^aphical subspecies, or
the constancy of such plants as hrackea {Pteridium aquilinum)
or Dryas odopetala as gainst the great variability of field pansy
(Viola tricolor, sensu lato), or chickweed (Stellaria media, sensu
lato) . It may be, of course, that the rpstriction of actual variability
here depends on quite other causes than a restriction of the
potentiality of variation. Much work must be done on the
subject before we can do more than guess.

hi any case we must beware of arguing that the inability of
specialized forms to produce new types must be due to an
inherent lack of genetic variability. This assertion is often made,
but cannot be upheld. We have already seen (p. 500) that the
failure Hes in the difficulty for selection of utilizing any varia-
tions except those tending towards further specialization in the
same direction. When the biomechanical limit has been reached,
the type is stuck and can do nothing but either maintain itself
or else become extinrt. However, as we have seen (pp. 324 seq.),
such limits are relative to the environmental situation: if this is
radically altered, evolutionary radiation may again set in, showing
that the previous standstill was not due to lack of genetic vari-
ability. The astonishing range of types produced by man in
domesticated animals, ev^ those like p%eons whose origin is
free frpm the suspicion of hybridization, conclusively demon-
strates the same fact. From another ai^le, the reserve of genetic
variability, much of it waiting to be elicited by selection, is
demonstrably enormous in most wild species (R. A. Fisher,
I930fl,p.96).

More to the point, are the examples of restricted types of
variation fpund in nature. Many groups appear to vary readily
in certain Erections, with dfficulty in others.

The rarity of greens in adult butterflies (and, to a lesser extent,
moths) is a case in point. In most other insect groups, green is a
common colour; and in view of its selective value as protective
(cryptic) coloration against a background of vegetation, this is
what one would expect. Indeed numerous larval Lepidoptera
show a green coloration which is obviously cryptic. The rarity
of green in the adults is all the more remarkable.

' ' ETOLtlTION V ■’XHE MOBEEN' SYNTHB:SIS .

Then among woodpeckers, reds, blacks, and whites
qwent, and yellows, greens, and browns may occur, but blues
appear to be unknown. GuUs, on the oAer hand, show *
exclusively a combination of white widi grey-blue or black:
reds, yellows, and greens are never found in their plumage
(though they occur frequently in their beaks or legs). Pcngmns,
^ain, show no red in dieir plumage, though some have yellow.
This is of interest, since Levick (1914) found that red posse^d
some special stimulating quahty for die Adelie Pengim. s
species is much addicted to the theft of the stones which consti-
tute its nest-material. Levick painted stones of difierent colours,
and found that the red ones travelled by theft across the colony
much faster than diose of any other colour. One niay presume
that red plumage would have an advantage in sexud selection;
but the bird’s plumage has remained black and white.

Here again we must beware of arguing that because certain
characters are normally not found, dierefore they cannot be
produced. The pierines or white butterflies provide a good
example to die contrary. As everyone knows, the prevailing
colour of these is white, often with black or greenish markings,
and sometimes with yellow or orange. In the Old World they
are practically restricted to this range of colour, and to certain
types of pattern. In South America, however, a number of
pierines have become mimetic: and these, to copy their models,
have developed a number of new colours and patterns not else-
where found in the group. Even in these, however, there is
some restriction, for all the pigments employed are pterins,
belonging to the katabolic type of substances produced from the
breakdown products of metabolism: no flavones, for instance,
occur.in pierines," save in one aberrant New World group (Ford,
i94oii). Thus the group appears to be subject to a restriction as
to t!ic chemical nature of its pigments, though in respect of its
patterns and to a certain extent of its actual coloration it must
be regarded as conservative rather than as compulsorily hmited.

Again, at first sight it might be supposed that the lower
mammals (all groups except Primates) were genetically restricted
as regards pigmentation, since they are confined to black, wliite.

BVOLUTIONARY TRENDS

519

gtey, brown, russet, and yellow, while in Primates scarlets,
pinks, blues, and greens are also found. This, however, would
appear to be a case of consequential evolution (see p. 525), the
greater range of colours among Primates' being a consequence
of their acquisition (alone among mammalian groups, apparently)
of colour-vision. A relevant fact is that, as I am informed by
Dr. S. Zuckerman, the red of buttocks and occasionally of face
in Primates is due, as in our own lips, to blood showing through
their skin. This device for producing visible red would have
been available to members of other mammahan groups, but
would have been useless in the absence of colour-vision.

There are some cases, however, in which certain variations
appear to be impossible or at best very difficult to produce. In
spite of intensive and long-continued efforts, breeders have
faded to give the world blue roses or black tulips. A bluish-
purple in the rose and a deep bronze in the tulip are the limits
reached: true blue and jet black have proved impossible.

We refer later to the small amount of new variation to be
foimd in the introduced English sparrow (Passer domestkus) in
the U.S.A. This was recently confirmed by Lack (1940c). No
marked local races have been established, and the variance of
individual populations has been scarcely or not at aU increased.
This rather surprising failure to vary may possibly be due to
lack of time (p. 521).

Restriction of variability may also be due to quite other causes,
namely to a lack of what is called modificational plasticity — ^the
capacity to react by modification to difierences in the environ-
ment (see p. 441). Various botanists (e.g. TiirriU, 1936; and see
Marsden-Jones and Turrill, 1938) have, shown that diferent
species of plants differ enormously in this respect, some, which
we may c^ stenoplastic, remaining extremely constant under a
wide range of environmental conditions, others, the euryplastic
types, reacting by marked changes in size, habit, proportions,
etc. p. 444). We have less information on the subject in animals.

T his phenomenon is of great interest ecologically and in
relation to minor systematks. We do not, however, know
whether it is corrected with any difference in actual or potential

520 evolution: the modern synthesis
genetic variabiHty, and this alone will have long-range cvolu-

“'S to an allied problem, that of the great ina-

bility of certain species as opposed to the relative mvanab.h^
of others. We have already touched on this m connection wi

the subject of polymorphism (p. 516). . , , 1

Hornell (1917), after detailed study of the lamclhbr^ch
Meretrix in Indian seas, concludes that the three speacs “^vo vc
differ markedly in their type of variabiHty. M. mretnx md M.
cOtenmta are very variable in colour, but 1

adult shape and size, whereas M. casta is highly vanable both
geographically and locally in these last respects.

Restriction of variation is sometimes only apparent Thus the
snails of the genus Cepaea, such as C. hortensis and_C. nemoraUs
(see p. 202), appear at first sight to be far inore vanable in their
ground-colour and banding pattern than the common garden
snail Helix aspersa. However, as Mr. Diver has pomted out to
me, the sheU of the garden snail is heavily suffused with a general
brown pigment, wHch masks any underlying variation. Actually,
it would seem that variation in these concealed patterns is just

as great as in the readijy visible patterns of Cepaea.

Similarly, there are two North American species of the ameih-
branch Donax, of which one (D. gmldii) is superfiaaUy vety
uniform, while the other (D. variabilis) owes its name to the
striking variation which it exhibits (Anon., 1941)- Examination
of the illustration, however, seems to show that the apparent
restriction of variabiHty in D. gouldii is due to a gener^ diminu-
tion in the intensity of pigmentation, which renders the various

patterns much fainter. r ^ 1

Bateson {1913, pp. M seq.) gives a number of other examples.
In some cases, however, if he had gone further into the subject,
the facts would not seem so curious. For instance, he cites the
case of two closely related British noctuid motlis of the genus
Diantkoecia, both common and wide-ranging; D. capsmcola
shows little variation, wliilc D. carpophaga “exhibits a complex
series of varieties”. He further mentions that the common
“Silver Y”, Plmia gamma, shows little variation in the mark

EVOLUTIONARY- TRENDS 521

frooi wliidi it takes its name, wliilc the corresponding mark in
P, interrogatiottis is $o variable that no two specimens are alike.
However, in the latter ease, he omits to nienticm that in ground
colour the two forms both show considerable variation, showing
that P. gamma is not stable as a species.

With Dianthoecky he has neglected the ecology of the two
species. Although he states that they arc similar in their habits,
this is not true in one important respect, for Mr. E. B. Ford
tells me that while the adults of D. capsincola rest in concealed
situations at the base of herbage, those of D. carpopkagay pre-
dominantly a coastal species, tend to rest on exposed soil and
rock. Their coloration is thus subject to selection for protective
reasons, and the variation to which Bateson refers is mainly a
regional one, forms from different localities being adapted to
the prevalent colour of the local background. For instance on
the south coast of Britain, whitish forms arc found in the chalk
areas, but beyond these, to the west, brownish types predominate.

These examples will serve to show the complexity of the
problem, and the danger of hasty conclusions. None the less,
some of Bateson’s eases seem to satisfy all rcquiremcJits. In
Britain, for instance, the pheasant stock (if we disregard re-
combinational variation due to crossing) is less variable than the
red grouse, in spite of the fact that the former has been intro-
duced into alien surroundings. In the United States, the introduced
house sparrow appears to be much less variable geographically
than many indigenous species (p. 519; J- C. Phillips, 1915)- How-
ever, the lapse of time may not have been sufficient to elicit
geographical differentiation (see p, 194), for this appears to
depend on selection as well as on inherent variabihty. In any
ease the species shows plenty of geographical variation in the
Old World.

At the moment we can give no explanation, whether in terms
of intrinsic nature or external selection-pressure, to account for
the restriction of variability in some species as against others of
the same genus, although we may say with some assurance that
some species seem to show a greater readiness to vary genetically
than do others, and further that a given type may produce

R-^

522 EVOLUTION : THE MODERN SYNTHESIS

certain kinds of mutational effects more frequently, others with
great rarity or perhaps never. On the other hand, thcorctica
considerations show that evolutionary change will still occur in
spite of wide differences in general .mutation-rate, provided that
selection is operative. Thus it will be rare that lack of evolu-
tionary change can be due to lack of raw materials in the shape

of mutations. . ,

So much for the possibihty of a restriction of the raw materiai
of variation, through the differential frequency of mutation in
various directions. Another restriction, of much more frequent
occurrence, is that of the utilizabihty of variation, through a
differential effect upon the selective value of mutations in dittcrcnt
directions. The former depends upon inherent properties of die
germinal material; the latter upon the past history of the species,
as embodied m its present organization, and upon its environ-
ment. We have given examples of the relativity of evolutionary
change to environmental conditions (p. 430). As an example of
past history limiting the advantageous directions of cliange,* we
have already considered the effect of past speciaUzation in
favouring further change in the same direction and inhibiting
it in other directions. The principle is, however, of wider
application. Once a given structural plan has been evolved, it
will be much simpler (I use a shorthand mode of expression)
to alter its parts quantitatively or to adapt it to new functions
than to evolve new organs. For this reason, the great majority
of evolutionary changes of structure consist in- changes of pro-
portion only, one part or organ being enlarged, another reduced.
To take-a striking example, the adult echinoderms have never
succeeded in escaping fully from their radial symmetry. Asym-
metrical and bilateral forms have been evolved, but never full
bilaterality with development of a head. Numerous other
examples of structures altering their function during evolution
and of the past dictating the hmits for the future (see p. 500),
will readily occur to the mind. An interesting minor one is the
fact that in groups with sporadic hearing, the evolution of this
capacity in conjunction with that of functional sound-production
may be followed by the evolution of a second distinct nietluid

BVOtUTIONARY TRENDS $21

of sound-production. TMs has occurred in several longicorn
beetles; and in one {Plagithysmus) two subsidiary methods have
been evolved (see the Cambridge Hatural History for details).
Numerous other examples of specialization in one type of sense-
organ being followed in evolution by a scries of allacstlictic
acquisitions designed to stimulate that particular sense-organ,
will occur to all biologists. Such restrictions, however, should
strictly not be called ordiogcnctic. They are radrer to be con-
sidered as cases of orthoseicctivc evolution (p. 500), but conse-
quential in die long range or historical sense (p. 545).

The fact remains diat evolutionary change is not completely
at random. In the first place it is restricted environmentally, hi
saying this we are only reaffirming the fundamental Darwinian
postulate of selection, for selection is always relative to the
environment, both inorganic and biological. This relativity, how-
ever, is so basic that it is often' neglected: its importance is thrust
upon our notice only when a chmatic revolution takes place,
or, more frequently, when there is some alteration in the bio-
logical environment, as with the colonization of new areas
where the balance of competitors or enemies is different.

It is, however, also restricted on accotmt of peculiarities in the
evolving organisms. Such internal restriction operates in two
ways, orthogenetically and historically. Both types of restriction
may 'play either a dominant or a subsidiary role in evolution.
The historical restrictions depend on the previous evolutionary
history of the stock and its effects on the machinery of selection.
Dominant historical restrictions arise from what we may call
the groove effect (p. 500), which Plate termed orthoselcction:
once adaptive specialization has begun in one direction it must
become progressively harder, on the basis of the knovra facts
of mutation, for selection to switch the trend onto another
direction. The result is an apparent orthogenesis. Subsidiary (or
consequential) historical restrictions simply make it easier for
selection to act in certain ways than in others, while leaving the
adaptive direction to be guided by selection.

A special case of subsidiary historical restriction is provided
by the Baldwin 'an d Lloyd-Morgan principle of Organic Selec-

524 EVOLtFTION : THE MODERN SYNTHESIS

tion, according to whicli an organism may in the first instance
become adapted to an ecological niche merely by behaviour
(vsrhether genetic or purely habitual) and any consequent non-
heritable modifications, after which mutations for the kind of
stmctural change suitable to the particular mode of life will
have a better chance of being selected. Where the modifications
are extensive, the process of their replacements by mutations
may closely simulate lamarckism {pp. 114, 296, 304). The principle
is an important one which would appear to have been unduly
neglected by recent evolutionists.

Trae orthogenetic restriction depends on a restriction of the
type and quantity of genetic variation. When dominant it pre-
scribes. the direction of evolution: when subsidiary it merely
limits its possibilities.

Dominant historical restriction is common, dominant ortho- ■
genetic restriction very rare, if indeed it exists at all. Subsidiary
historical restriction is common. It may be important in barring
certain major lines of advance, but allows considerable freedom
in the direction of adaptive specialization. Subsidiary orthogenetic
restriction is probably frequent, but we are not yet able to be
sure in most cases whether a limitation of variation as actually
found in a group is due to a limitation in the supply of mutations
or to selection, or to other causes. It is, however, certain that
some mutational effects recur regularly in some allied species,
and probable that this phenomenon is widespread. This last
fact may contribute to parallel evolution — a type of direc-
tional change fix which ortliogenetic and selectionist agencies
are combined.

To sum up, the only important agency restricting the direction
of evolutionary change is the historical one, leading to a purely
apparent orthogenesis. The subsidiary restrictions are truly sub-
sidiary, in that the supply of variation remains sufficient to
allow a degree of freedom in die direction of change which
is always considerable and in certain cases at least appears
to be, for all practical purposes of adaptive speciaHzation,
unlimited.

EVOLUTIONARY TRENDS 535

6. CONSEQUENTIAL EVOLUTION: THE CONSEQUENCES OF
DIFFERENTTAL DEVELOPMENT

Under this head we may discuss types of trend which are
initiated or maintained with special readiness as a consequence
of the way in which genes operate to produce their effect during
development.

Let us begin with an example neatly worked out by Haldane
(1932a, p. 124; see also Castle, 1932), which demonstrates how
the results of selection at one period of the life-cycle may have
repercussions on other periods and affect the species and its
evolution in unexpected ways. The phenomenon with which he
deals is that of intra-uterine selection in mammals which are
poiytocous, i.e. bring forth a number of young at one birth.
Here there must be air intense pre-natal selection, since a con-
siderable percentage of every Htter dies in^utero. Rapidity of
growth especially must be at a premium, since space and nutrition
are Hmited, and any advantage gained by an embryo estabhshing
itself early is likely to be of permanent advantage throughout
the critical stages.

Haldane suggests with some plausibility that any rapidity of
pre-natal growth thus acquired is likely to be transferred in whole
or in part to post-natal life as well, and that intra-uterine selection
may thus help to account for the progressive increase in size seen
in so many mammalian lines during their evolution.*

At any rate, the converse seems to hold good, namely that on

* This cannot be the only factor responsible for such trends towards evolu-
tionary increase in bulk. For one thing, size-increase (up to a certain limit)
must often be directly advantageous in its own right; and for another," the
phenomenon occurs ako in other types, such as reptiles, in which no phase of
intra-uterine existent is passed through. It might be interesting to compare the
rate of evolutionary size-increase in monotocous and poiytocous placental types;
but we could never be sure at what period a type which at present is mono-
tocous had ceased to have litters of several young. Haldane hiiiiself in a later
work (1938, p. IZ5) points out that a similar trend towards increased size will
operate in polygamous species in which the males fight for the females. In the
first case^ intersexual selection will operate to increase the size of the males;
and then some of this increase in size will tend automatically to be transferred
to the females (c£ Winterbottom, 1929 and 1932), so that the size of both sexes
will tend to be pushed beyond the optimum, or what would be the optimum
for othor reasons.

526 evolution: the moobkn synthesis

account of intra-uterine selection it ^

polytocous mammal to slow down its rate

of ic most characteristic features of man, and one by winch Ins
capacity of learning is utilized to the Mest extent, is F^ciss y
such a slowii^ down of general rate of development Without
it he could not in all probabiHty have become^lully human or
biologically dominant. Judged by the law (which applies to most
other niaimals so far investigated) Wh regulates Ae amount
of food consumed before Ae adult phase b reaped, mans
immaturity has been lengthened some sevenfold. This could not
have occurred m a polytocous form. It was only after inan s
ancestors ceased to have Htters and began to brmg forth a smgie
young at a birA that Ae furAer evolution of man became

this general slowmg down had numerous corollaries.
The typical adult human conAtion of hair on Ae head btrt
dmost complete abence. of hail on the body is passed through
as a temporary conAtion at about Ae time of buA by Ae anthro-
poid ape. The hymen of Ae human female has been stated to
represent the persistence of what m lower mammals is an
embryonic stage m Ae development of Ae urm^gemtal system.
Most sttiking of all, Ae general form of the humm face and
skull, wiA its absence of snout and of bony ridges on Ae cramum,
is qmte similar to Aat of Ae foetal or newborn ape, but quite
dissinAar to that of Ae adAt (see p. 555 ; and Bolk, 1926).

The general slowmg down of man’s post-natal development
is doubtless due m part to its possessing selective advantage. But,
as HAdane pomts out, it may also be m part Ae mdhect carry-
over from , a slowing of pre-natal development. In the drcnni-
stances of anthropoid apes and of primitive sub-man a foetus is
on the whole better nourished and less exposed to danger Aan
a newborn infant, so Aat pre-natal slowing, wiA consequent
prolongation of Ae mtra-uterine phase, is here advantageous m

* The slowing is already marked in anthropoid apes, but not so extreme as in
man, Spence and Yerkes (.1937) sHow ^that whereas the

crease inbnlk indomesticmammals and rodents vanes from 400 to 1,200 per cent
per annumduring the juvenile period; in the chimpanzee it is 21-27 per cen ,
and in man about 10 per cent.

EVOLUTIONARY TRENDS

527

polytocous mammals. (The non-black eye- and hair-colours
(except red hair) of certain human ethnic groups appear also to
be due to a slowing of the processes concerned with melanin-
deposition: McConaiU and Ralphs, 1937.)

This prolongation of a more protected early phase may also
apply to the larval period, for instance in insects with their
coenogenetic larvae, which are often highly adapted to their
secondary mode of life. One need only think of the mayfly widi
its imaginal phase reduced both in structure and in duration.

Sometimes this reduction is carried to its logical extreme, and
the adult phase is wiped out of the life-history by neoteny. This
has demonstrably occurred in various beetles, and in the axolotl.
It has probably taken place in ourselves as well: there is every
reason to suppose that our adult ancestors possessed heavy brow
ridges and protruding jaws, and that our smooth foreheads and
orthognathous faces represent primarily the prolongation into
maturity of a foetal and neo-natal phase that we share with
the apes.

Haldane in an interesting paper (i932i») discusses tliese and
similar phenomena from the standpoint of tlic time of action of
the genes controlling them. A more comprehensive view, how-
ever, sucluas that adopted by de Beer (1940a), would include as
stiU more important the genes’ rate of action.

As A. R. Moore (1910, 1912) first suggested, and as Gold-
schmidt (summarized 1927), Ford and Huxley (summarized
1929), and others have conclusively shown, a large number
(possibly the majority) of genes exert their effects through tlie
intermediation of a process operating at a defiaiite rate. They may
be the direct cause of the process, or they may influence the
rate of a process originated in some other way: in either case
mutations in the genes concerned will alter the rate of some
process of development.

The speeds of processes which such rate-factors control are
not absolute, but relative — relative to tlie speed of other processes
of development and of development .in general. Furtlier, it is
found that a decrease in the rate of a visible process is in general
accompanied by a delay in the time of its initial onset, and vice

528 evolution: the modern synthesis^ ,. ^

versa. This may be merely a threshold efe hut
important biological consequences, since it will aftect the durattm

of any Other characters which can only manifest themselves

before the process visibly manifests itseE _

Furthermore, such processes do not necessary contoue
indefinitely. Often they reach an equilibrium; when this is so
the final level of the equilibrium also appears to be correlated
with the rate of the process. This is so, for instance, mth eye-
colour in Gammarus (Ford and Huxley, 1929) and probably m
man. The physiological basis of this fact is obscure, but once
more it may involve interesting biological consequences, m
addition to such rate-genes, others are known which appear only
to affect the time of onset of a process and not its rate.

Attempts have been made by representatives of the Morgan
school (see e.g. Schultz, 1935) to minimize the importance of
these discoveries, by asserting that they comtitute only a
re-description of old phenomena and add nothing truly new.
On the cbntrary, I would maintain that the concept of rate-genes
is as important for biology as is the concept of genic balance or
the gene-complex. I need not go into its bearings upon physio-
logical genetics— the problem of how the genes become translated
into characters — save to say that it has in this field already proved
itself an indispensable tool. Here we are concerned with its
evolutionary implications (see also the excellent discussion in

Goldschmidt, 1940, pp- 311 seq.).

In the first place, since rate-genes are common, it is a legitimate
provisional assumption diat the rates of developmental processes
in general are gene-controlled. Further, the simplification intro-
duced into an analysis of development by the concept of relative
rates of processes — exemplified by work such as Goldschmidt s
on intersexuality and other problems (summarized 1927).
Huxley’s (1932) on the proportion of parts in animals. Ford and
Huxley’s (1929) on rate-genes, and Sinnott’s (i 935 ; Sinnott and
Dunn, 1932, p. 341) on the role of rate-genes in determining
fruit-shape in plants — makes it desirable to try this. key first of
all when attacking any developmental problem.

Next, as Swinnertoa (1932) has stressed, a progressive nuita-

EVOLUTIONARY TRENDS 529

tional change in the speed of processes controlled by rate-genes
affords a complete formal explanation of many paleontological
data, e.g. in various moUuscan shells.

It further affords an explanation certainly of most and prob-
ably of all cases of so-called reversal of dominance. Hie classical
example is that recorded by A. Lang (1908) in crosses between
red- and yellow-sheUed snails. The Fa when young showed a
ratio of 3 yellow : i red, whefeas in the older individuals the
ratio was 3 red : i yellow. The explanation is that all those
individuals with either one or two “red” genes eventually
become red, but the rate at which this occurs is reduced when
the gene is present in single dose. For other examples sec p. 218;
Goldschmidt (1927); Huxley and Ford (1929).

It also helps us to understand the presence, the persistence, and
the variability of vestigial organs. I may here cite a previous
discussion of the subject (Huxley, 1932, p. 235):

“As regards vestigial organs, the arm-chair critic often demands
of the evolutionist how the last stages in their reduction could
occur through selection, and why, if reduction has gone as far
as it has, it could not go on to total disappearance. In the light
of our knowledge of relative growth, we may retort that we
would expect the organ to be formed of normal or only slightly
reduced relative size at its first origin, but then to be rendered
vestigial in the adult by being endowed with negative heterogony
[allometry: see Huxley and Teissier, 1936].* If rate-genes are as
common as they appear to be, then what we have called the
line of biological least resistance would be to produce adult
vestigiality of an organ, by reducing its growth-coefficient. So
long as it is reduced to the requisite degree of insignificance at
birth (or at whatever period a larger bulk would be deleterious),
there is no need for reduction of its growth-rate to be pressed
furdier. But the negative heterogony with which it is endowed
will continue to operate, and it will dicrefore continue to grow

* Needham and Lerner (1941) have now proposed the term heterauxesis to
supersede allometry in cases of true relative growth, reserving the latter term
for comparison of relative proportions in different types. It will probably be best
to use alhnwrphosis for this latter use, keeping allometry as a general and iudusivc
term (Needham, Huxley and Lerner, 1941).

530 evolution: the mooern synthesis

relatively smaller with increase of absolute size. This last fact
may account for the apparently useless degree of reduction seen
in some vestigial organs, e.g. that of the whale’s hind-limb. The
degree of reduction may be useless considered in relation to the
adult, but the relative size in the adult may be merely a secondary
result of the degree of negative heterogony needed to get the
organ out of the way, so to speak, before birth. In addition
threshold mechanisms will possibly be at work, so that the organ,
after progressive reduction, eventually disappears entirely.

“In such cases quite small differences in growth-ratio, if the
range of absolute size over which they operate is considerable,
will make quite large diferences in final relative size, a fact
which indubitably will help to account for the high variability
of vestigial organs. Even when the organ itself never grows, as
in the imaginal structures of insects with a metamorphosis, a
similar degree of variability may be brought about by relatively
small variations in the rate-genes responsible.”

Consideration of the threshold-effect of any genes acting as
rate-controllers for vestigial organs will also show that such
organs must be unusually variable (op. cit., pp. 236-7).

The concept of rate-genes indeed provides a great simplification
of the facts of recapitulation and anti-recapitulation. Whenever
the rate of a process is correlated with time of onset and with
final equilibrium-ievel, a mutation causing an increase in rate
will produce recapitulatory phenomena — ^it will drive the visible
onset of the process further back in ontogeny, will add a new
“hypermorphic” character at the end of the process, and will
cause all the steps of the original process to be recapitulated, but
in an abbreviated form, during the course of the new process.
This will account, for instance, for many of the recapitulatory
phenomena seen in the suture-lines of ammonites (p. 507).

Conversely, a mutation causing a decrease in rate will have
anti-recapitulatory effects— it wiQ prolong die previous phase
longer in ontogeny, it will not only slow the process down but
render it “hypomorphic” by stopping it at a lower level of
completion, and it-vi^ remove certain previous adult characters
and push them off the time-map of the Ufe-history (sec Huxley,

EVOLUTIONARY TRENDS

531

1932, pp. 239-40). Many of the phenomena of so-called "racial
senescence” in ammonites, iacluding the gradual uncoiHng of
the shell, may be due to phenomena of this type (p. 507)-
Swinnerton (1938) has given numerous examples of processes
of both types revealed in actual fossil lineages.

Haldme (1933) has drawn attention to a still further conse-
quence of these facts, coupled with Fisher’s principle of the origin
of dominance (p. 75). He begins with a reminder of Goldschmidt’s
generalization tliat dominant alleles tend to promote not only a
greater intensity of action, but one with a greater range both in
space, over the organism’s body, and in time, during its develop-
jnent. (This "greater range in time” is a less accurate formulation
of the principle of earlier onset of rate-genes promoting a greater
speed of process, as found by Ford and Huxley, 1929.) This, he
then points out, will mean that even when homozygote and
heterozygote are alike in the final stages there will be an early
period in which the process involved is more advanced in the
homozygote. Thus, “in so far as developmental abnormality is
disadvantageous, the Fisher effect will always be tending to
increase the activity of the genes”, and so extendmg their action
further and further back into ontogeny. 'Where tire form of
early stages is closely adaptive, as it must be in larvae, this back-
ward spread of gene-effects concerned with adult characters will
be checked by natural selection.* But where there is a sheltered
embryo, its form will have litde survival value, and the process
will tend to continue unchecked. This would promote phenomena
of tachygenesis and recapitulation, for many genes would tend
originally to come into action rather late, but gradually to
extend their activity back into ontogeny, so that the phylo-
genetically- older characters of the adult would tend to
manifest themselves earlier in development, and this would
be more prominent in forms with embryos than in those
with larvae. This tendency may explain why recapitulatory
phenomena appear to be commoner than anti-recapitulatory.

* This conclusion is borne out by the fact mentioned by Ford (1937) that it
is rare in Lepidopteta for mutations to aifcct the visible character of both the
larva and the imago.

532 evolution: the modern synthesis

Castle (1932). from his data on rabbits, has drawn general
evolutionary conclusions similar to Haldane’s. _

As de Beer {1930) has pointed out, when coenogenetic chang^
occur in the embryo or larva, the adult remaining unchanged,
neither paleontology nor comparative anatomy would register
any phylogenetic advance. But if now neoteny or foetalization
occurs, the old adult characters may be swept off the map and
be replaced by characters of a quite novel type.

This process, which he calls clandestine evolution, has been
operative on a small scale in neotenous beetles and amphibia.
Garstang (1922} has suggested that it has operated on a large
scale in the ancestry of the vertebrates and of the gastropods. It
is in any case a principle of far-reaching importance.

A clear-cut example comes from the species of the snail Cepaea.

It seems quite plain that their non-banded varieties are produced
not because they contain a gene causing the total absence of
pigment, but because they contain one which slow-s down
pigment-formation and delays its visible onset relatively to
general growth, to such an extent that growth is completed
Wfore any pigment can be formed.

This is a comparatively unimportant effect; but when major
processes are affected, such as metamorphosis, sexual maturity,
or general rate of growth or development, the results may be
far-reaching. Paedogenesis is caused by relative acceleration of
the processes leading to sexual maturity. Neoteny in the axolotl
and presumably in insects is due to the slowing down of the
processes leading to metamorphosis. The condition seen in man
should not strictly be called neoteny, but rather foetalization or
perhaps JuveniMzation: this would seem to be produced by a
general slowing of developmental rate, relative both to time and
to sexual maturity.

The existence of rate-factors has a bearing upon the problem
presented by apparently useless characters. For alterations in the
rate of a process will often automatically produce a number of
secondary and apparendy irrelevant effects. These will persist
if they arc harmless, or if any harmful effect is more than counter-
balanced by the favourable effect of the initial change; and once

EVOLUTIONARY TRENDS ^533

produced they may of course become utilized for other purposes.
Numerous examples of such “correlated characters”, as Darwin
called them, arc now known (pp. i 88 seq.).

I will take a simple example from Gammarus. The depth of
colour of the eye depends essentially upon the rate of deposition
of melanin in an origmaUy pure red eye. But the visible efect
depends also on the size of the eye at a given time— when the
eye is smaller, the melanin is more crowded and the eye looks
darker (Ford and Huxley, 1929). lu point of fact depth of eye-
colour has been found to be modified, first by genes controlling
the rate of melanin-formation, secondly by genes controlling
relative eye--site, and diirdly by genes controlling the rate of
development of the whole organism. Thus a mutation affecting
the relative rate of eye-growth will alter the depth of eye-
pigmentation.

It would seem inevitable that many of the apparently useless
features used in diagnosing species are correlated characters of
this type. This may well prove to be the case with many of the
pigmentary and other visible characters of the subspecies of
Lymantria (p. 216; Goldschmidt, 1934). In cotton species {Gossy-
pium) flower-colour, apparently owing to some imderlying
metabolic difference, has coroUa-pize as a correlate (Silow, 1941).

Of course not all useless “correlated characters” need be
dependent simply on alterations in the rate of a process. The
white-eye series of mutants in Drosophila also cause alterations in
the shape of the spermatheca and the colour of tlie testis-sheath.
Ford’s analysis (1930) of Dobzhansky’s data has made it probable
that while the eye-characters of the series have been selected
against to make their expression recessive, no selection has been
operative on the internal characters (p. 80), which would then
'be mere correlates. Even here, however, it is probable that the
different eye-colours of the white series represent the cross-
sections of a series of rate-curves.

Important examples of correlated characters arc the higlicr
mental faculties of man. It is obvious that natural selection cannot
have been operative directly in bringing about the evolution of
intense musical or mathematical ability, or indeed of many

534 liVOI.UTION: THE MODERN SYNTHESIS

specifically human faculties. As H. S. Harrisoiv ( 193^0 puts it,
wSng aJ an anthropologist, “it seems clear, mdeed, that wha -
Tvi Ictors were concerned in the ancient evolution of the

modern type of man, the upper limits of his pow

of mind were not determined by the struggle lor existence .
Natural selection, however, could be and doubtless was operate
in bringing about the evolution of speech and conceptual thought
with their corollaries of rational control in the practical sphere
and freedom of association between the
of mental powers. Once, however, this level of mental Jt,
ment was reached, the so-caUed higher faculties immediately
became possible. They arc implicit in the general type of biaii
^gSon required for speech and conceptual f
J therefore correlated characters in our i.nse. A soinei^ t
similar case from lower organisins is that

doubtcdly true song has imprtant functions, f ® ^

threat and advertisement (Huxley, 1938^)- But given the complex
emotional make-up of song-birds, song is uttered m many circum-
stances where it has other functions or is even functionlcss,
produced “for its own sake”. The sedge-warbler {Acroccplmlus
schoenobaenus) will sing as an expression of .mger. Many birds
sing as an expression of general wcU-being; the autumn recrud-
escence of song in many species would seem to be due to tins,
and to have no function. The vocal miimcry of many birds
would seem to be an entirely unsclcctcd resultant, wholly com-
parable to human higher faculties.

A peculiar correlated character is diat of himian scapular
shaped convex iimer border to the shoulder-blade being corre-
lated with general fitness and high expectation of life, and vice
versa for a concave one (Graves, 1932)- Here, however, t ic

correlation is a comparatively low one.

The development of correlated characters durmg evolution
,uay stimulate orthogenesis One of the most apparently con-
vincing bits of evidence for die reality of orthogenesis was the
discovery of Osborn (1929) and his school, that horns of the
same type were evolved independently, in the same region o .
the skull, in four separate groups of titanodiercs. Sturtevant

EVOLUTIONARY TRENDS 535

(1924)’, however, suggested that characters of this sort might be
correlated characters, and the study of relative growth (Huxley,
1924; 1932, p. 218) has provided a simple explanatory basis for
this view in this particular case. The horns of titanotheres are,
like most horns, aUometric, increasing in relative size with the
absolute size of the animal, and not appearing at all below a
certain absolute size. We have only to postulate the potentiality
of frontal horns in the ancestral titanothere stock, for their
independent actualization in the different groups to become
inevitable so soon as a certain threshold of body-size is reached.
Increase of body-size is probably advantageous up to a limit; if so
the horns are the useless correlate of a useful character. It would be
more accurate to say initially useless, since presumably once they
appeared they were employed in fighting* (and sec footnotef).

The interesting analysis of Hersh (1934) has shovra that
evolutionary allometry can be quantitatively studied. Thus in
titanotheres, the evolutionary development of hom-length
relative to basilar skuU-length obeys the law of simple allometry,
but with an unusually liigh equilibrium-constant or partition-
coefficient (a= about 9‘o), He has furdier pointed out that,
provided no change in growth-mechanism occurs during geo-
logical time, the equilibrium-constant for the relative growth of
an organ will be the same for the evolution of a stock as for die
development of a single individual within die stock
Extrapolation of his curve indicates that the primitive titan-
otheres of the Eocene should have horns about o- 5 mm. long — in
other words, of inappreciable size; and this is actually die case.

He also records the important fact that the equilibrium-
constants for the relative growth of certain characters (e.g.
zygomatic width and free nasal length relative to skuh-lengdi)

* The faa that in rhmexx^roses, horns appeared independently in three separate
h'nes, but on different regions of the head, is not to be explained cither ortho-
geneticaliy or on the basis of simple allometry as in titanotheres. An allometric
factor must presumably be involved, but also, it would appear, a selective factor.

f Hersh points out that while the horns originally appeared as correlated
characters, presumably as a result of selection for increased general bulk, once
they were established and were of use in fighting, selection for increased horn-
size might occur, and would then bring about increased bulk as a “correlated
character” in its turn.

MODERN SYNTHESIS

may change at definite points in geological time, indicating
changes (presumably mutational) in the underlying ontogenetic
growth-mechanism at certain stages in the evolution of the group.

According to Robb (1935-6) face-length in horses shows the
same growth-constant for its ontogenetic and phylogenetic
allometry, so that tire phylogenetic change in skull-proportions
would be entirely consequential on general size-increase. How-
ever, Reeve and Murray (1942) have shown that this is incorrect,
the growth-constant changing during ontopny from 1-5 to
!.o, while the phylogenetic growth-constant for the more primi-
tive genera is 1.8; thus in modern (hypsodont) horses, lengthening
of the lace has been anticipated in early embryonic life.
Robb further maintains that whereas digit's ii and iv show
the same slightly negative phylogenetic allometry in 3-toed
and i-tocd forms, there is an abrupt change in the constant (i)
defining the initial size of the primordia, Hersh’s work on titano-
thercs indicates similar abrupt changes in the relative growth-
constant a, as dex's Reeve and Murray's on the horse's face; while
Herzberg and Masslcr’s (1940) bn rodent incisors indicates a
gradual increase in a during phylogcny. Thus studies on relative
growth sometimes lay bare die genetic mechanisms underlying
evolution. Similarly, related subspecies may differ eidier in
their box a values for certain allomctric organs— e.g. the antennae
of the ainphipod Corophitm volutator (Chevais, 1937).

Detailed studies as to the different rate of change with time of
various characters involved in a trend, such as that of Swinnerton
(1921) on carboniferous corals, arc likely to throw considerable
light on selection and on consequential evolution.

Allomctric growth is also without doubt the explanation for
Lamecre’s and Geoffrey Smith’s rule, namely diat a large number
of organs, all of them apparently allonietric in the individual,
tend to be of larger relative size in those species or genera of a
group which arc of greater absolute size. This is most clearly
shown in beetles, but appears also to exist in hornbllls, antcaters,
and other forms (see Chainpy, 1924; Huxley, 1932, p. 212).

This principle has obvious taxonomic bearings. In the first
place, percentage measurements of die proportionate size of an

EVOLUTIONARY TRENDS

537

organ will have no diagnostic value unless either the organ is
isometric, or there is not only a fixed adult phase (as in insects
or higher vertebrates), but one with a restricted adult si2e-range
(which is not true in many insects, e.g. Lucanidae, and even
some mammals, e.g. the red deer {Cervus elaphus), as discussed
by Huxley (193a, pp. 42 and 205). For some of the taxonomic
implications, see Klauber (1938) on relative head-length in
rattlesnakes, and Swinnerton (1940) for shell-shape in Ostrea-
Gryphaealmsages (pp. 508, 515 h.).

A recent important study of this question has been made by
Reeve (1940) on the anteaters of the family Myrmecophagidae.
These include three well-marked genera, Cyclopes, Tamandm,
and Myrmecophaga, characterized by increasing size and increasing
relative face-length, measurable by facial index (see Table).

Adult

Skull-length.

Fadal Index.

Growth-coefficient (a)
of Maxillary Region.

Cyclopes . ,

4*5-5 cm.

0*5

1-26

Tamandua . .

13-14 cm.

0-8

1-36

Myrmecophaga

36-38 cm.

1*6

1-77

The facial index is the ratio of maxiUa length to rest-of-skuU
length. Cyclopes has a distinctly short face, while in the other
two genera the snout region is very obviously elongated, excep-
tionally so in Myrmecophaga, where the maxilla is over ii times
as long as the rest of the skuU.

The degree of aUometry in the snout was then found for each
species by comparing skulls of diSerent absolute sizes. It will be
seen from the table that all three genera show positive aUometry,
though it is intensified with increase or absolute size.* This last
feature is unusual, but is one which might be anticipated where
we are dealing with stages in a trend towards a particular special-
ization.

Reeve shows that most, but not aU, the differences in facial

* Tlie difference between the growth-coefBcient in Cyclopes and Tammdua is
only doubtfully significant.

538 evolution: the modern synthesis

proportion and skull structure between the three genera, especially
between the two small ones, are purely consequential on the
differences in absolute size; and there is no ground whatever for
the proposals that have been made by various systematists to
erect a separate subfamily or even family for the reception of
Cyclops, on the ground of its diftcrent facial proportions.

IntraspecificaUy, too, the allometry principle has interesting
taxonomic implications. Thus the genus Tatnandua has been
divided by recent systematists into two species, including nine
subspecira, many of which latter have been erected on the basis
of differences in percentage snout-length. Reeve in a further
analysis (1941) shows that this procedure is invalid, since many
such percentage diSerences are purely consequential on not very
large differences in absolute size. This entirely bears out the
warning given by Huxley (1932, p. 204, etc.) as to the preference
of taxonomists for employing percentage rather than absolute
size-differences in diagnosis. In any case, to erect subspecies on
a few skulls (sometimes only one) which happen to show slight
differences in proportion from the type, as has been done by
e.g. Lonnberg (1937), is bound to lead to confusion. Doubtless
geographical subspeciation will have occurred in these wide-
ranging animals; but to establish the subspecies properly, absolute
measurements, allometric constants, and pelage characters njust
be taken into account as well as diSerences in proportion.

Allometry has applications even to craniometrical indices. In
mammals increased dolicocephaly appears to accompany in-
creased absolute skull-size (Kappers, 1928). In man, this may also
hold, though the evidence is more definite for increase of relative
skull-height with absolute size (see Huxley, 1932, p. 220).

A fact of considerable interest is that certain organs, notably
the vertebrate brain, show diferent degrees of allometry intra-
and inter-specifically (see fid! discussion in de Beer, i94o<i). In
the simple allometry formula, brain-weight = b (body-weight)",
the intraspecific value of the equiHbrium-constant o lies between
0* 22 and 0' 27. This appean to be a consequence of the develop-
mental facts that neuron-number is approximately constant
within the species, and that increase of body-volume causes an

EVOLUTIONARY TRENDS

539

increase of neuron-volume which is somewhat less than propor-
tioml to the 1/3 power of body-volume, or in other words to
the linear dimensions of the body, apparently on account of the
linear increase in axon-length necessitated by increased body-size.

A further fact is that the value of the equilibrium-constant for
different-sized individuals is lower in domestic races than in
wild species (though, of course, the size-range of the domestic
forms will be much greater). Thus in wild Canidae the value
is about 0*26, in the domestic dog 0*22. This may perhaps be
correlated with the different type of selection operating in the
two cases, that for domestic races here being essentially concerned
with size, irrespective of detailed physiological adjustments of
the brain to a particular size.

Interspecifically, the equilibrium-constant is more than twice
•as high, about o* 56. For diis to be the case, it is necessary that
more neurons as well as larger neurons should be present. The
brain-volume is thus nearly proportional to the surface of the
body. This must represent the optimal relation physiologically.

Finally, there is the curious fact that the constant of initial
proportion b varies from one group of species to another by
whole-number multiples of \/2. Dubois has suggested that this
is due to the cerebral neuroblasts undergoing a different number
of cell-divisions before finally differentiating. Whether this
be true or not, we may be sure that the fact is consequential
upon some ontogenetic process.

Lumer (1940) has successfully applied allometric analysis to
die classification of the domestic breeds of dog (Cants familiaris).
This enabled him in the fim place to rule out the great majority
of earlier classifications as being based solely on adult proportions
(percentage ratios of various measurements). By plotting various
absolute adult measurements of different-sized breeds on a double
logarithmic scale, he obtained evolutionary growth-constants,
as Hersh did with the titanotheres (p. 535), and he was then
able to group the various breeds into six "allometric tribes”,
each characterized by possessing a particular set of growth-
coefficients.

Di&rent tribes may show the same growth-coefficients for

540 evoiution: the modern synthesis

certam organs. Thus, for example, the terrier tribe (Alsatian,
setter, poodle, fox- 4 errier, etc.), the bulldog tribe, the Great
Dane tribe (with St. Bernard and Newfoundland) and the
greyhound tribe (Borzoi, greyhound, whippet, etc.), have the
following growth-coefScicnts (a), representing the skull’s growth-
relations in length and in breadth respectively.

Tetriers. Bulldc^. Great Danes. Greyhounds.
Snout laigth/lowcr jaw lengdi l-il 1*72 0*69 t'll

Palate width/palate kngdi . . 0’69 0*24 I-I2 o*6o

The greyhounds share with the terriers the length-relations of
the snout, and presumably diverged later in respect of their width-
relations. The other two groups have become more extreme in
both relations, but in opposite directions.

The measurements on wolves {Cams lupus) are interesting.
For the snout-length/cranial-lei^th relation, the wolf stands at
the intersection of the two main curves on to which all the
forms (except the toy terrier) fall. For other measurements it
usually conforms to the terrier or the Great Dane type. Though
some of the results must be regarded as tentative, it seems clear
that the form of domestic breeds is dependent on two main
factors — ^first, mutations affecting the growth-coefficients of
particular regions, and secondly, changes in proportions conse-
quential on changes in absolute size. These changes will, of
course, be quite diferent in the various tribes because of their
difference in growth-coefficients.

Size in snakes is correlated with the pattern of the scales. As-
this is used for taxonomic diagnosis, the consequential effects
of size-changes may be of systematic importance. Thus Stull
(1940) describes inter-group dines in the genus Pituophis, involv-
ing a progressive diminution in the number of scale-rows in
passing outwards from the centre of distribution. In addition,
other scale-characters are graded and the relative tad-length
increases markedly. But all these chararters appear to be directly
consequential on decreased absolute length. In fishes {Catastomus)
delay in development relative to growth leads to an increased
number of scales (Hubbs, 1941).

EVOLUTIONARY TRENDS 541

It is in general fair to state that change in absolute size is almost
certain to produce numerous correlated changes in proportions.
It is also true that change in relative size of an organ is quite
likely to be accompanied by correlated changes in various of
its own characters; this fact is illustrated by the antlers of deer
(Cervidae) and the mandibles of stagbeetles (Lucanidae), where
it has important taxonomic consequences in denying taxonomic
validity to groups distinguished on the basis of the form of
aEometric organs (see Huxley, 1932, p. 204 seq.). In addition,
continued increase in absolute size will so increase the relative
size of an organ with well-marked allometry that it will even-
tually approach the boundary of disadvantage. Selection may
then operate to reduce its rate of growth and therefore its final
siae, or, if conditions alter rapidly, the organism may be caught
napping in an evolutionary sense, and be extinguished. Such
considerations would account for such apparent cases of ortho-
genesis as the antlers of the Irish elk and the fantastic horns of
some beedes (see Champy, 1924), as well as the limited size of
certain types, such as the fiddler-crabs {Uca), where males
weighing 17 g. have large claws three-quarters as heavy as the
rest of the bedy (Huxley, 1932, pp. 32, 216).

Hie principle also has practical implications, as to which I may
again quote from a previous publication (Huxley, 1932, p. 88);

“Hammond (1928, see also 1921) has also shown that the growth-
gradients in the limbs and elsewhere affect the muscles as well as the
bones, so that the study is of practical as well as theoretical impor-
tance. An important point made by Hammond may be given in
his own words-

“ ‘As the animal grows, it changes its conformation; at birth
the calf or lamb is all head and legs, its body is short and shallow,
and the buttocks and loin are comparatively undeveloped; but, as
it grows, the latter — ^buttocks, loin, etc.— grow at a fester rate than
the head and legs, and so the proportions of the animal change. . . .
The extent to which these proportions change determines its con-
formation; those which develop most for their age have the best
meat conformation, while those which develop least have the
wont. . . . Breed improvement for meat, therefore, means push-
ing a stage fiuther the natural chaise of proportion as the animal

542 evolution: THE MODERN SYNTHESIS

matures. . . . The adult wild mouflon ewe is in its proportions

but little in advance of the unproved SulFolk lamb at birth, although

it is much larger.’

“Thus it would appear that one of the chief advances made by
man in creating improved breeds of sheep and other meat animals
has been simply to steepen growth-gradients which already operate
during post-nata development in the wild ancestral forms. Hammond
himseu (1928) has expressed a similar idea: ‘The improver of meat-
producing animals has apparently not chosen mutations occurring in
isolated points independently, but rather has based his selection on
the generalized correlated changes of growth.’ ”

McMcekan (1940-1) gives a similar analysis for the pig.

Another case where alteration in the rate of processes may
have results of taxonomic importance is found in flying-tfish
(Exocoetidae). In certain species barbels are only present in young
specimens. But the’ size at which the barbel is lost varies very
greatly, apparendy owing to the time-relations of the process
diScring in different subspecies (Brunn, 1933, who quotes similar
cases from other fish).

A possible further consequential evolutionary result of mechan-
isms regulating the proportion of parts I owe to a suggestion by
Mr. Moy-Thomas. La standard textbooks it is customary to
classify the extinct group of Palaeoniscoid fishes into two separate
groups, the Palaeoniscidae and the Platysomidae. The only
essential distinction between the two, however, is one of body-
form, the former being elongated, the latter short and deep in
body. In the course of their history, the two groups show parallel
evolutionary trends.

If the groups were truly distinct, of separate origin, this would
be a remarkable case of parallel evolution. On the other hand,
as D’Arcy Thompson (1917, Chap. 17) first showed, differences
in body-form of even greater extent than those between these
two groups can be brought about by quite simple geometrical
transformations, and Huxley (1932) pointed out that the actual
mechanisms of relative growth, in the shape of growth-gradients
or growth-fields of relatively simple conformation, provide a
biological basis for such transformations, since alteration in a
growth-gradient would affect the proportionate size of all the

EVOLUTIONARY TRENDS

543

parts in whose growth it was concerned (see also Goldschmidt,
1940, pp. 311 seq.).

An alternative hypothesis, therefore, is that the Palaconiscidae
and the Platysomidae in reality constitute but one natural group,
and that in every epoch two main types, elongate and deep-
bodied, were evolved in relation to different modes of life. Only
further study can decide between these two alternatives.

Granted the basic growth-mechanism responsible for the spiral
shells of gastropods (Huxley, 1932, Chap. 5), only a limited
set of shell-forms is available. Rensch (1934, p. 89) has collected
interesting examples of the extraordinary convergence produced
by this determinism of growth-mechanism in land-snails.

The claim that the concept of rate-genes is as important as
that of the gene-complex would thus seem to be justified. With-
out the concept of the gene-complex we could obtain little
insight into the intricate phenomena of genic balance or the
puzzles of the evolution of dominance and recessiveness. Similarly
the study of developmental processes controlled by rate-genes
has illuminated the reversal of dominance, and the evolutionary
aspects of recapitulation, of neoteny, foetalization, clandestine
evolution, and apparently useless characters, as well as helping
to a simpler understanding of the innumerable cases of quanti-
tative evolution.

7. OTHER CONSEQUENTIAL EVOLUTIONARY TRENDS

So far, we have been considering mainly the evolutionary
effects of differences in rate of development, whether between
different species, diferent variants of the same species, or different
parts of the body. However, there are many other examples of
consequential evolution. Let us begin with one firom bony fish,
which has been discussed by Moy-Thomas (1938). Here, the
dermal bones of the skull appear to be determined primarily in
relation to the system of sensory canals. Bones not formed in
direct relation with the canal system are produced to fill gaps
between the canals. The precise number of centres ojcratiye in
such a gap varies (in relation to fectors at present unknown, but

544 evoxtjtion: the modern synthesis

partly in relation to the size of the gap), so that the parietal
region, for insmce, may be occupiea by bones varying from one
to four in number. The canal system is on the whole constant in
plan throughout the class, but varies in the detail of position of

its various parts. . , t

This win cause variation in the Hmits of parttcular bones, m
the total number of bones determined in relation to the canal
system, and in the size of the gaps to be filled by other bones.
The parietal gap, for instance, is in some fish so much reduced
that there is no room for a separate centre in it, and the parietal
region is filled by a canal-determined bone, the supra-temporal.

It is obvious that, in these circumstances, the classical concept
of homology breab down. We cannot expect to homologize
individual bones throughout the class (see also WestoU, 1936)-
The evolution of the dermal bones of the fish s skull is entirely
consequential on the changes in detailed pattern of the sensory
ranal system. The interminable disputes of morphologists brought
up in the post-Darwinian school, determined to discover precise
correspondence between individual bones and to draw phylo-
genetic conclusions from their homologies, turn out to have no
factual basis. The right answer was difficult to find for the simple
reason that the wrong question was asked. We are reminded of
the fact pointed out by Jacques Loeb (see Loeb, 1912) that in
embryo fish (Fundulus) the wandering pigment-cells eventually
arrange theniselv® along the blood-vessels, so that the visible
colour-pattern foUmvs the pattern of the circulatory system.

In the Malagasy insectivore Hemicentetes semispirtosus an appar-
ently adaptive reduction in tooth-size (p. 287) has certain
consequential efiects on skull-fbrni (Buder, 1941)- fr would
be interesting to see whether such effects are general.

An example of great evolutionary importance is that cited by
Watson (1926) of the locomotion of vertebrates. Among fish,
there are two main types of locomotion — diat of most teleosts,
in which movement is mainly restricted to the base of the tad,
and that of various other forms, in which the whole body is
markedly undulated. Iii this second group, the elasmobranclis
hold their pectoral fins stiffly out, while the Dipnoi and Pelyp-

EVOLUTIONARY TRENDS 545

terns and its relatives do not. Only from this last sub-type could
the locomotion of the tailed Amphibia be derived. These were
first aquatic, but even later tlieir locomotion was “a svamming
upon land” (but see the criticism of Moy-Thomas, 1934). This
is an excellent example of what we may call historical evolu-
tionary consequence, where the past history of an organism
helps to determine its future mode of evolution. Some examples
of this we have already mentioned (p. 522) under the head of
historical restriction of variability. A striking case of diis in the
evolution of our own species is the effect of monotocy (p. 525).
Our own evolution also provides an example of rather a different
nature. The assumption of the erect posture at once converted
many of our internal adjustnients into maladjustments. Here was
an immediate consequential step ; the incipient counteraction of
these maladjustments is a further one.

Sex has numerous consequential effects. In the first place, there
is the tendency for characters acquired by one sex, e.g. by intra-
sexual seleaion, to be transferred in whole or in part to the
other (see Meisenheimer, 1921, chap. 23; Winterbottom, 1929,
1932). This will in certain groups increase die amount of evolu-
tionary diversification to be found between species. Conversely,
the difference in interaal environment provided by the two
sexes may and frequendy will give rise to sex-limited characters
which are wholly non-adaptive at dicir origin, but may later
be used as the basis for adaptive (e.g. epigamic) sex-limited
characters (cf. discussion in C. and F. Gordon, 1940, who suc-
ceeded in building up stocks of Drosophila with a non-adaptive
but definite sex-limited female character — brown palp).

Somewhat similarly, the sex-limited difference in hair-number
varies considerably in different species of Drosophila (Mather,
1941). Thus in D. melanogaster and in D. simulans females have
rather more hairs than males, but in D. virilis many fewer.

A very extraordinary case concerns the external genitalia of
hyenas (see L. H. Matthews, 1939b). These are indistinguishable
externally in males and non-parous females. Copulation, as a
result, appears to be an eljiborate and difficult feat. Matthews

546 evolution; THE MODERN SYNTHESIS

quence of some unusual upset of endocrine balance, the females
having an excess of androgenic substances ^

deficiency of oestrogens. Tlie condition is closely parallel to that
seen in adrenal virilism in females of our own speaes.

Empirical observation reveals numerous odier peculianties _ ^
organic construction wWch may form the basis for consequentia

evolutionary trends. . . u, •

In mammals, for example, the extremities ( points . ears
limbs, muzzle, and tail), either as a whole or in their ternimal
portions, are frequently of a different colour from the rest of t e
body. This undoubtedly depends on a physiological pe^anty
of 4cse regions, namely their lower temperature. Det^ed
studies have been made of the problem in the Himalayan breed
of domestic rabbit, which is white with black pomts (see Ujin,
1931 ). The Himalayan pattern depends on an allele of the albino
series which reduces the intensity of melanin-production. At t is
level of production, melanin can only be formed m regions
below a certain critical temperature. In normal animals, these
regions exist only in the points; but by experimental prow ure
(shaving and subsequent exposure to cold) black hair can be in-
Led L any region of the body. Thyrotoion also affects the
reaction.The Siamese pattern in cats is similar (Ujmandlljm, I930h

In the most general terms, the points provide a differential
environment for the manifestation of pigmentation-genes, an
when these arc working close to a threshold level of productmn,
differential effects are readily produced in these areas. The
quantitative restriction of this or that type of pigrnentation
depends on quantitative reduction in the activity of tte genes
responsible; but its localized distribution depends on a differential
pattern in the construction of the organism, providing different
opportunities for gene-expression in different areas. ■

The dorsal stripe present in so many breeds and species of
mammals is doubtless a further example of the same principle.
Numerous other examples may be found enumerated in works
such as those ofHaecker (i925> I92'7)-*

* It is necessary on theoretical grounds to draw a sharp distinction between
such cases, dependent on the general construction of the organism, and others

rsVOL0TIONARY TRENDS

547

The presence of such organizational patterns will result in a
considerable amount of parallel evolution in regard to visible
colour-pattern.

Mammalian extremities (points) also react to temperature in
another way, namely by enlarging at higher temperatures
(p. 213). This is again very likely due to the increase in their
heat-loss and the lowering of their intrinsic temperature at lower
external temperatures. In any case, this will account for a large
number of parallel trends (character-gradients) affecting relative
size of extremities, which arc found in nature.

Tl)c most extensive type of organismal pattern is the organic
gradient or as it is better styled gradient-field (Huxley, 193.'^)-
Although we are still in ignorance as to tlie physiological basis
of such gradients, they undoubtedly exist, and by providing
differential environments for gene-expression, open the door to
consequential evolution. Such gradients may be total, extending
through the whole organism, or partial, extending through a
single organ or region.

The interesting effect of different regional gradients on pig-
mentation is well shown in the zebras, in which the striping is
always at right angles to the main axis of the region, whether
trunk or limb. Where hind-hmb area meets trunk area, inter-
action of the stripe system occurs, giving curious patterns. These
patterns differ from species to species (see Hacckcr, 1927), doubt-
less on account of slight differences in the form and relative
intensity of the underlying gradients.

One of the most widespread results of the existence of such
gradients is the common type of coloration in many vertebrates,
which produces counter-shading with dark back and light belly,
following the main dorso-vcntral gradient of the embryo.
According to the mode of gene-expression, the dark may grade
into the light or be sharply delimited, and according to the

which depend o« the existence of local fieIds-~-*e.g. the sharply delimited phiniagc-
fidds of many birds (Haecker, op, cit.). These appear to have more analogy
with the localized morphogenetic fields into which the developing einbryi^
becomes divided, and which may persist into the adult, ns revealed by regenera-
tion experiments in urodclcs (references in Huxley and dc Beer t934> Huxley;

1935)*

548 EVOIUTION; THE MODEKN

threshold of gene-activity, the light ventral ar.
or smaller. Genetic analysis in rodents has re^
alleles, whose differential effect within tins gi

tatively different. L ^ '' ■it i

uniformity of coloration from mid-dorsal to

may be established, :

pattern may be reversed, in r.v

^lecats and the well-defended ratels (Mephb

Ictonyx, and MelUvord), in order to c

conspicuousness, or rc]
but the existence of
great deal of parallel evolution in

The phenomenon known as u-

depends undoubtedly
gradieni

By al tering the threshold of gene-activity,

^ - •< ' 1 mid-ventral line

aad by special mechamsms the normal

the offensive skunks and Cape
■ tis, Ompatus,

enhance instead of to reduce

:placedby quite different types of patterns;
the gradient has provided the basis for a
-'--t characters.

determinate variability also

on the existence of organismal or regional
■ - Its For instance, in the ladybird beetle Malta frigida
fearapkin, 1930) aU stages occur ftom ompotted through spotKd
to nearly urhfotm bUck types. There is, however, a tegulanty
in the order in which the seven pairs of spots appear and m that
in which they arc subsequently joined. The gradicnt-ncld appears^
to be a complex one, and there is accordingly a certain amount ot

variabiUty, but the general regularity is marked.

Cause (1930) has made an interesting comparative study of the
subject in tliree species of the coleopteran genus Phytodecta. M
most commonly have five pairs of spots m a characteristic pattern
on the elytra, but variants occur, especially in the mmus direction.
Variability in spot-number is least in P. mfipes, greatest m
R ptmindh, with P. Unmeams in an intermediate position. This
difference, however, depends on some relation between an
antcro-posterior gradient and the threshold for pigment-deposi-
tion in the spot areas, since in aU forms die anterior spots are
rarely (or never) absent, but the posterior spots frequendy, and
increasingly so with increasing distance from the ante'rior end.
The threshold for invariable pigment-deposition (spot present
in 99 per cent of cases or over) is halfway down the elytra in
P. rufipes, so that three pairs of spots are always present; in
P. linmeanus it is near the anterior end, leaving two stable pairs,
and in P. viminalis sail mote anterior, leaving only one invariable

EVOLDTIONARY TRENDS 549

pair of spots. This may be due eitlier to alteration in the slope
of the gradient or in the intensity of pigment-formation, or both.
That the gradient is really a gradient-field and capable of altera-
tions affecting spot-pattern is shown by a comparison of spot-
frequency in P. linnaeanus and P. vimimlis. Whereas in die
latter the facts can only be interpreted as the basis of a uniform
gradient ranning diagonally from the external anterior margin
to the posterior point of junction of the elytra, the gradient of
P. linnaeanus must be more complex, starting as in P. viminalis,
but in the posterior half of the elytra running out towards the
external margin again.

Thus the form of the gradient-field in the elytra has bodi
general and special consequences, for the intra- as wcU as die
inter-specific variation of the pigmentation of die genus.

A related phenomenon occurs in the pluteus larvae of sea-
urchins. The skeleton of the plutei belonging to various cchinoids
of extremely different adult structure, and assigned to different
suborders or even subclasses, is virtually identical. Von Ubisch
(1933) suggests, on the basis of experimental analysis, that this
is due to the existence of a general type of gradient-field deter-
mining skeleton-formation, shared by most typical plutei, and
that simple quantitative alterations in this would bring about
strong similarity in skeleton, irrespective of common descent or
adult resemblance, dius simulating orthogenesis. In a later paper
(1939) he shows that cytoplasmic viscosity is the chief agent
affecting the form of the larval skeleton. By treatments altering
viscosity, normally simple skeletons can be made more complex,
and then show a close resemblance to the normally complex
skeletons of other fomis.

A slightly different phenomenon of the same sort occurs in
another ladybird, Epilachna chrysomelma (Zarapkin and H. A.
Tiniofceff-Ressovsky, 1932). Here the shape of single spots was
studied. It was found that with increase in absolute spot-size
(antero-posterior length) most spots became increasingly elong-
ated in form (liigher ratio of length to brcaddi). The degree to
which this occurs, however, is much greater in some spots than
in others, and may differ markedly even in neighbouring spots.

550 evolution; the modebn synthesis

Hie gradient-field affecting pigment-deposition must accordingly

be distorted in different ways in different regions.

A genetic difference in spot-size will therefore brmg about
consequential diferences in spot-form: such a difference was
found to distinguish the races from Palestine and Corfu respec-
tively. One spot (near the hind end of the elytra) was found to
behave in a more complex manner, becoming first more and
later less elongate with increase in absolute length.

Similar studies made by R. H. Johnson (1910) on the entiie
feniily Coccinellidae, have shown that some intrinsic plan of
organization (gradient-field) has important consequential effects
on the evolution of pattern in the whole group of ladybird
beetles. A great volume of data on this and cognate subjects is
discussed by Vogt and Vogt (1938).

Interesting work has also been done by Schwanwitsch (I924^
1926) on the patterns of butterflies’ win^. He shows that in a
large section of the Rhopalocera, all existing patterns can be
derived from an original prototype through the modification of
different markings by a limited number of methods. Both the
existence of the original prototypic pattern and the Hmited
modes of its alteration operate to restrict the evolution of pattern
in the group in a consequential way.

Returning from colour-patterns to other characters, we find
that the existence of the abnormal condition of the head brown
as otocephaly is in guinea-pigs due to a combination of genetic
and environmental factors acting upon the primary gradient of
the embryo (or that of the organizer). Similarly the suppression
of digits in the course of evolution in the guinea-pig family, and
their subsequent restoration by selective breeding (p. 501),
appears not to have depended on genes acting on each digit
separately or directly, but on genes afecting the general tendency
of the limb primordium to break up into discontinuous parts
(digits) at its distal end, by interfering with a controlling centre
of the digit-forming field, situated on the post-axial side of the
hand region of the limb-bud (discussion of bodi cases in S.
Wright, 1934^, and of the latter in J. P. Scott, 1938). For instance,
the same gene which in single dose tends to restore a normal

EVOLUTIONARY TRENDS

551

thumb, and often a normal little toe also, in double dose is lethal,
but permits development to a stage at which the embryo is
seen to possess the rudiments of 8 to 12 toes per foot. Further,
one rnodifier was found which promoted the development of
thumbs but inhibited the development of little toes. This may
most readily be interpreted as a gene steepening a gradient
concerned with digit-separation, and running from the pre-axial
to the post-axial side of the Hmb.

An interesting consequence of serial repetition of structures
such as teeth is mentioned by Gregory (1936). The mammalian
tooth-series of course early becomes differentiated into markedly
distinct subseries. But the fundamental scriation, with its capacity
for more extended repetition, remains, and when a character
is added in one subseries, ^‘'as in the case of new cusps in the
premolars or new cuspuJes in die molars, the whole tooth-row
often tends to be glossed over, so to speak, with the same surface-
features, so that all the cheek-teeth, as in the horses, come to look
amazingly like each other”. This phenomenon Gregory calls
^^secondary polyisomerism”; it frequently imparts a quite decep-
tive appearance of lack of diSerentiation, the new features which
have spread over a large part of the series disguising the older
characters differentiating the subseries.

A curious consequential effect is the weakening of feather-
structure associated with the presence of red lipochrome pigment.
This appears to be due to the inhibition of feather-differentiation
by lipochrome (Desselberger, 1930). The chief result is the
reduction or loss of barbules, wliile the barbs fail to show full
difierentiation into cortical and medullary layers.

In the barbets of the genus Lybius, black-, red-^ and white-
headed forms are found. One of the last-mentioned, L. torquatus
zQtnbae, studied by Salomonsen (1938), appears to have been
recently derived by mutation (of at least two genes) from a red
form (see p: 195).

Red feathers, as we have seen, become worn much more
rapidly and thoroughly than black; but the white feathers of
zombae are so weak that they are almost pathological, the whole
white portion rapidly disappearing with wear. Apparently, the

552

evolution: the mooern synthesis

prcsaice of lipodirome confers a certain degree of mechani<^
solidity. Thus in L. t. zombae, the white feathers are doubly
weakfthey retain Ac weak structure characteristic of Ae red
feaAers from which Aey have arisen, and also, through the lack
of aU iipochrome, Ae remaining structure is further weakened,
la certain, other white-headed ; LybinSf however, the white
feathers are normal Presumably oAer mutations have occurred
which restore Ac normal feaAer-structure. Salomonsen notes
Aat some mdividuals of L t. zombae have patches of normd
white feathers on Ae head; possibly selection is already at work
repairing some of Ae deleterious effects of Ae white mutations
(cf. Ae similar “repair” of Ae St. Bernard dog; p. 71)- ^ ^ .

After this chapter was writtea, Goldschaiidt (1940) paHisi^d
his Material Basis of Evolution, m which he pays considerable
attention to the problem, devoting over 100 pages to evoktioa
and Ae potentiaHties of development”. Already m 1920 he had
recognized Aat “a change m Ae hercAtary type can occur only
wiAin Ac possibAdes and limitatiom set by Ae normal process
of development”, and had Austrated Ae point at some length.
In this latter work he restates Ac matter more positively, e.g.
p. 322 : “What is called in a general way the ipechanics of develop-
ment wA decide Ae Arecrion of possible evolutionary changes.
In many cases Acre wA be oAy one Arection. This is orAo-
genesis without .Lamarckism, without mystidsm*. . . ^ ^

Among his examples we may cite a few. Where xertamTed
pigments normally occur in Lepidoptera, yelow varieties (awr-
rations) occur, and white mutants may . arise from the yellow
forms (p. 12). This, however, .is due to alteration in the rate and
intensity of red pigment*-for.matipn (Ford, 1937)

He... agrees, that. the demonstrarioii,, of, grow/th-gradients. and
growth-fields : accounts for many . examples of .non-adaptive
variation, and lightens die burden on. natural selection by showing
that numerous correlated changes in proportions, wil be expected
to occur as the r^ult .cT single , mutatfom affecting, the fo
foegrowfo-gra

■ * It is wortli recalling timt ■ckvelopmcntal pmcesses '^lgiiten^^^^^
:;mtural.sd€cd6ii^’ in a number of oUicr ways,.tbongb to.by

EVOLUTIONARY TRENDS

553

Some of his most striking cases concern the morphogenetic
effect of the ductless glands. For instance, once the thyroid has
been evolved, certain changes in it will be expected to exert
similar consequences in numerous types. It is no accident that
the thyroid is associated with metamorphosis not only in Am-
phibia but in various fish such as eels, flounders, and mud-hoppers
(Periophthalmus). In the last-named, the aquatic larva becomes ,an
amphibious adult, but excess thyroid causes an intensification of
all its adaptations to aerial existence, most notably in the pectoral
fins, which come to simulate a tetrapod limb (p. 277; Harms, 1934).

Agam, achondroplasia and other pecuHarities in size or propor-
tions, which are certainly or probably dependent on endocrine
changes and which occur as aberrations in man and other forms
and as breed-types in dogs, goldfish, etc., are closely similar to
the normal condition in various wild species (short-legged
carnivores, bulldog-faced fish, etc.). There is at least a prima
facie case for regarding the primary change leading to the evo-
lution of these species as being similar to that involved in the
production of the pecuHar breeds and aberrations. On the other
hand, there is no reason to suppose that the change in die wild
species must have been abrupt, as Goldschmidt assumes. It is
more likely to have been a gradual process, accompanied by
buffering with modifiers (cf. our discussion of Stockard’s results
with St. Bernard dogs, p. 71). It seems clear, however, that the
endocrine system constitutes a “chemical skeleton” whose exist-
ence and nature prescribes certain Hmils to, and certain favoured
modes of, evolutionary change in its possessors.

With the progress of what Haecker (1925) calls phenogenctics
and of physiological genetics in general, numerous other examples
will undoubtedly be uneardied in the most diverse groups of

cation, not by consequential effects of the type we have here been discussing.

I refer to the extraordinary functional adaptation of fine structure and often size
seen in bones, tendons, blood-vessels, etc. (see discussion in Huxley and de Beer,
1934, chap. 13, §§ 6, 7, pp. 431 seq,). These have finequently been held up as im-
possible of explanation on a selectionist view. So they would be if they were the
result of genetic adaptation; but all the details appear to be due to modifi-
cational a^ptation, produced anew by fimctional demands in each individuaL
The general framework is genetically adaptive, and so is the general capacity
for reaction; the rest is modificational polish.

554

evolution: the MOUERN SyNTHESIS

organisms. Wc are here concerned only to establish pnnciples.

It seems clear that the existence of orgaraxationai patterns m
organisms, whether in the shape of general, rcgiona , or oca
gradient-fields, or in some other form, will have consequential
evolutionary effects. It will for one thing account for a great °

otherwise mysterious parallel evolution, e.g. in pattern, in hom-de-

veiopment in titanotheres,in relative size of aUometric organs, etc
Aggregation, as in social hymenoptera, can also be regarded
as a type of organization, and may have important consequential
effects. To take but one example, the wood-eating habits ot
primitive termites, so important for their evolutionary success,
could, it seems, only have arisen in a social form. For their digestion
of cellulose depends on the presence of symbiotic protozoa; and
these are lost at each moult, so that reinfection can oidy occur
through association with other, non-moulting mdmduals (see
discussion in Emerson, 1939 )'

One might perhaps also include a category of Historical conse-
quential effects, as when types evolvecl in relation to one habitat
manage to invade another. Thus, as Professor Salisbury informs
me in a letter, various species of trees in the neotropical rain-
forests are deciduous; and all are closely related to deciduous
temperate types. However, this is perhaps to extend the concept
of consequential evolution too widely, until it becomes merged
in the obvious fact that in evolution the present and future of an
organic type is partly determined by its past.

Examples such as those of social insects (pp. 480, 482), o
certation in pollen (p. 481), of selection in abundant as against
rare species, and of intra-uterine selection in polytocous mammals,
show how the type and course of evolutionary trends may be
altered according to the type of competition and selection at
work. A somewhat similar consequential trend in this field
concerns the effect of inter-male competition in birds and other
groups. The result has been that in general the males have become
much more differentiated than the females, their secondary sexual
characters being usually striking and specifically distinctive; and
further that some of this masculine diversification lias then been
transferred to the females, although in them the characters are

EVOLUTIONARY TRENDS

555

functionless (see Darwin, 1871; Meisenlieimer, 1921, chap. 23;
Winterbottom, 1929 and 1932 ; Huxley, 1938a and andp. 545).

The examples we have been considering in these sections show'
how the Tact that most genes affect the rate, the time of onset,
the duration, and the type of developmental processes, wiU pro-
vide the raw material for trends involving progressive alteration
in one or other of these factors of development. Since the raw
material is so abundant, consequential trends of this sort will be
frequent. A description of some bearings of the subject is given
by Huxley (1932, Chap. 7), and fuller evolutionary discussion
is given, not only by Goldschmidt (see above), but by Haldane
in his previously cited paper (1932&), by de Beer (i94ol)),' and
from the standpoint of physiological genetics by Waddiogton
(1941 &). The course of Darwinian evolution is thus seen as deter-
mined (in varying degrees in different forms) not only by the
type of selection, not only by the frequency of mutation, not only
by the past history of the species, but also by the nature of the
developmental effects of genes and of the ontogenetic process in
general.

Postscript. — Weidenreich’s important recent paper (1941)
deals wnth consequential trends in mammalian skulls, dependent
upon brain-growth. The brain’s relative growth-rate is high in
early embryonic life; in ,most mammals, it later slows down
markedly, and the high allometry of the face then comes into
play. In dwarf domestic breeds and small wild species, facial
allometry is checked early. There normally results not only a
relative orthognathism, but also absence of cranial superstructure
(sagittal crest, supra-orbital ridges, etc.), persistent cranial sutures,
rounded palate, smaller teeth, often with simplified pattern,
relatively wide cranial cavity (brachycephaly), etc. ; in young and
dwarf dogs, the frontals are almost entirely cranial, while in
adult large dogs their major part is facial. Man, though not a
dwarf species, shows the “dwarf ’ ’ type of skull. This is not due
to the. retention of visible foetal characters, as postulated by
Bolk (p. 526), but to the persistence into later stages of the brain’s
early high relative growth-rate.

CHAPTER 10

evolutionary progress

1. Is evolutionary progress a scientific concept? . - • • P-

2. The definition of evolutionary progress P-559

3. The nature and mechanism of evolutionary progress . p. 562

4. The past course of evolutionary progress p. 5^9

5. Progress in the evolutionary future P- 57 ^

I. IS EVOLUTIONARY PROGRESS A SOENTIHC CONCEPT?

Hie question of evolutionary or biological progress remain.
There stUl exists a very great deal of confusion among biologists
on the subject. Indeed the con&sion appears to be greater among
professional biologists than among laymen. This is probably due
to the common human failing of not seeing the wood for the
trees j there are so many more trees for the professional!*

The chief objections that have been made to employing
progress at all as a biological term, and to the use of its correlates
higher and lower as applied to groups of organisms, are as follows.
R^st, it is objected that a bacillus, a jellyfish, or a tapeworm is as
well adapted to its environment as a bird, an ant, or a man, and
that therefore it is incorrect to speak of the latter as higher than
the former, and illogical to speak of the proc^ leading to
their production as involving progress. An even simpler objection
is to use mere survival as criterion of biological value, instead of
adaptation. Man survives: but so does the tubercle badilus. So
why caE man the i%her org?nism of the two ?

A somewhat similar argument points to foe fact that evolution,
both in foe fossE record and indirectly shows us numerous
examples of specialization leading to increased efficiency of
adaptation to this or foat mode of life; but that many of such

* For a.Mler discwsdoii of certain aspects of the problem see Huxley, 1923a,
1936, 1940; Wells, Hiodey/andWelis, 1930, Book 5, chap. ,6, § 5.

EYOLUTIONAHY PROGBESS

557

Specialized lines become extinct, while most of the remainder
reach an equilibrium and show no jfiirtber change.

This type of objection, then, points to certain fundamental
attributes -of living things or their evolution, uses them as defini-
tions of progress, and then denies that progress exists because
they are found in all kinds of organisms, and not only in those
that the beHevers in the existence of progress would caE pro-
gressive.

A slightly less uncompromising attitude is taken up by those
who admit that-there has been an increase of complexity or an
increase in degree of organization, but deny that this ^ any
value, biological or otherwise, and accordingly refuse to dignify
this trend by a term such as progress, with al its implications.

Some sociologists, faced with the problem of reconciling the
objective criteria of the physical sciences with the value criteria
with which the sociological data confronts them, take refuge in
the ostrich-like attitude of refusing to recognize any scale of
values. Thus Doob in a recent book (1940) wrkes:

“In this way, the anthropologist has attempted to remove the
idea of progress from his discipline. For him, there is just change,
or perhaps a tendency towards increasing complexity. Neither
change nor complexity is good or bad; diere are differences in
degree, not in quality or virtue. ... The sweep of historical
progress reveals no progressive trend. ...”

By introducing certain objective criteria into our definition of
progress, as we do in the succeeding section, this objection can
be overcome, at least for pre-human evolution. In regard to
human evolution, however, as we shall see in the concluding
section of this chapter, the netde must be grasped, and human
values given a place aihong the criteria of human progress.

The second main type of objection consists in showing that
many processes of evolution are not progressive in any possible
sense of the word, and then drawing foe conclusion that progress
does not exist. For instance, many forms of life, of which foe
brachiopod Lingula is foe best-known example, have demon-
strably remained unchanged for enormous periods of several
hundreds of millions of years; if a Law of Progress exists, foe

558 evolution: THE MODERN SYNTHESIS

objectors argue, bow is it that such organisms are exempt from
its operations?

A variant of this objection is to draw attention to the numerous
cases where evolution has led to degeneration involving a
degradation of form and function, as in tapeworms, Sacmlina
and other parasites, in sea-squirts and other sedentary forms:
how, it is asked, can the evolutionary process be regarded as
progressive if it produces degeneration?

This category of objections can be readily disposed of Objectors
of this type have been guilty of setting up an Aunt Sally of their
own creation for the pleasure of knocking her down. They have
assumed that progress must be universal and compulsory: when
they find, quite correctly, that universal and compulsory progress
docs not exist, they state that they have proved that progress
does not exist. This, however, is an elementary fallacy. The task
before the biologist is not to define progress a priori, but to
proceed inductively to see whether he can or cannot find evidence
of a process which can legitimately be called progressive. It may
just as well prove to be partial as universal. Indeed, human
experience would encourage search along those lines; the fact
that man’s progress in mechanical arts, for instance, in one part
of the world is accompanied by complete stagnation or even
retrogression in other parts, is a familiar fact. Thus evolution may
perfectly well include progress widiout being progressive as a
whole.

The first category of objections, when considered closely, is
seen to rest upon a simil^ fallacy. Here again an Aunt Sally has
been set up. Progress is first defined in terms of certain properties :
and then the distribution of those properties among organisms
is shown not to be progressive.

These procedures would be laughable, if they were not lament-
able in arguing a lack of training in logical thought and scientific
procedure among biologists. Once more, the elementary fact
must be stressed that the only correct mediod of approach to the
problem is an inductive one. Even the hardoied Opponents of
the idea of biological progress find it difficult to avoid speaking
of higher and lower organisms, though they may salve their

2 . THE DEHNITION OF EVOLUTIONARY PROGRESS

Proceeding on these lines, we can immediately rule out certain
characters of organisms and their evolution from any definition
of biological progress. Adaptation and survival, for instance, arc
umversai, and are found just as much in ^lower^'’ as in "'higher^''
forms: indeed, many higher types have become extinct while
lower ones have survived. Complexity of organization or of
life-cycle cannot be ruled out so simply. High types arc on the
whole more complex than low. But many obviously low organ-
isms exliibit remarkable complexities, and, what is more cogent,
many very complex types have become extinct or have speedily
come to an evolutionary dead end.

Perhaps the most salient fact hi the evolutionary history of
life is the succession of what the paleontologist calls dominant
types.^ These arc characterized not only by a higli degree of
complexity for the epoch in which they lived, but by a capacity
for branching out into a multiplicity of forms. This radiation
seems always to be accompanied by the partial or even total
extinction of competing main types, and doubtless the one fact
is in large part directly correlated widi the other.

In the early Paleozoic the primitive relatives of the Crustacea
* For fuller summary, see Wells, Hushey, and Wells (1930), Book 5.

560 iiVOLUtiON : THE'' M, 015 :E 1 N SYNTIIESIS

known as the trilobites were the dominant group. These were
succeeded by the marine arachnoids called sea-scorpions or
eurypterids, and they in turn by the armoured but jawless
vertebrates, the ostracoderms, more closely related to lampreys
f ha" to true fish. The fish, however, were not far behind, an
soon became the dominant group. Meanwhile, groups both
from among the arthropods and the vertebrates became adapted
to land life, and towards the close of the Paleozoic, insects ^d
amphibians could both claim the title of dominant groups. TJc
amphibia shortly gave rise to the reptiles, much more fully
adapted to land life, and the primitive early insects produced
higher types, such as beetles, hymenoptera and lepidoptera.
Higher insects and reptiles were the dominant land groups in the
Mesozoic, while among aquatic forms the fish remained pre-
eminent, and evolved into more efficient types: from the end of
the Mesozoic onwards, however, they show little further change.

Birds and mammals began their career in the Mesozoic, but
only became dominant in the Cenozoic. The mammals continued
their evolution through the whole of this epoch, while the
insects reached a standstill soon after its beginning. Finally man’s
ancestral stock diverged, probably towards the middle of the
Cenozoic, but did not become dominant until the latter part of
tlic Icc A^c.

In these last two cases, the rise of the new type and the downfall
of the old was without question accompanied and facilitated by
world-wide climatic change, and this was probably true for
otlier biological revolutions, such as the rise of the reptiles to

doniinaixce^

When the facts conceming dominant groups are surveyed in
more detail; they yield various interesting conclusions. In the
first place, biologists are in substantial agreement as to what
were and what were not dominant groups. Secondiy, some
groups once dominant have become wholly extinguished; like
the trilobites, eurypterids and ostracoderms, while others survive
only in a much reduced form, many of their sub-groups having
been extinguished, as with the reptiles or the monotremes, or
their numbers enormously diminished, as with the larger non-

EVOLUTIONARY PROGRESS 56I

human placentals. Those which do not show reduction of one
or the other sort have remained to all intents and purposes
unchanged for a longer or shorter period of geological time, as
with the insects or the birds. Finally, later dominant groups
normally arise from an unspecialized line of an earlier dominant
group, as the birds and reptiles from among die early reptiles,
man from the primates among the mammals (p. 525, footnote).
They represent, in fact, one among many lines of adaptive
radiation; but they differ iSrom the others in containing the
potentiality of evolving so as to become dominant on a new level,
with the aid of new properties. Usually the new dominance is
marked by a fresh outburst of radiation: the only exception to
this rule is Man, a dominant type which shows negligible radiation
of the usual structurally-adapted sort, but makes up for its
absence by the complexity of his social life and his division of
labour.

If we then try to analyse the matter stiH further by examining
the characters which distinguish dominant from non-dominant
and earlier from later dominant groups, we shall find first of
all, efficiency in such matters as speed and the application of force
to overcome physical limitations. The eurypterids must have
been better swimmers than the trilobites, the fish, with their
muscular tails, much better than either; and the later fish are
clearly more efficient aquatic mechanisms than the earlier.
Similarly the earlier reptiles were heavy and clumsy, and quite
incapable of swift runnii^. Sense-organs also are improved, and
brains enlarged. In the latest stages the power of manipulation is
evolved. Through a combination of these various factors man
is able to deal with his environment in a greater variety of ways,
and to apply greater forces to its control, than any other organism.

Another set of characteristics concerns the internal environ-
ment. Lower marine organisms have blood or body-fluids
identical in saline concentrations with that of the seawater in
which tliey live ; and if tfib composition of their fluid environment
is changed, that of their blood changes correspondingly- The
higher fish, on die other hand, have the capacity of keeping
their internal environment chemically almost constant. Birds

563 evolution: THE MODERN SYNTHESIS

and mammals have gone a step further: they can keep the
temperature of their internal environment constant too, md so
are independent of a wide range of external temprature change.

The early land animak were faced with the problem or
becoming independent of changes in the moisture-content o
the air. This was accompUshed only very partiaUy by amphibia
but fully by adult reptiles and insects through the development of
a hard impermeable covering. The freeing of Ac young verte-
brate from dependence on water was more d^icult. The great
majority of amphibians are still aquatic for the earher part of
their existence: the elaborate arrangements for rendermg the
reptilian egg cleidoic Q. Needham, 1931, PP- ^2 scq.) were
needed to permit of the whole life-cycle becoming truly

tri sJi

There is no need to multiply examples. The distinguishing
characteristics of dominant groups all fall into one or other of
two types-rthosc making for greater control over the environ-
ment and those making for greater independence of changes m
the environment. Thus advance in these respects may pio-
vidonally be taken as the criterion of biological prisgress.

3. THE NATURE AND MECHANISM OF EVOLUTIONARY
PROGRESS

It is important to realize that progress, as thus defined, is not
the same as specialization. Specialization, as we have previously
noted, is an improvement in efficiency of adaptation^ for a
particular mode of life: progress is an improvement in efiiciciicy
of living in general. The latter is an all“roiind, the former a
one-sided advance. Wc must also remember that in evolntionary
history wc can and must judge by final results. And there is no
certain case on record of a line showing a high degree of special-
ization giving rise to a new type. All new types which tlieiiiselvcs
are capable of adaptive radiation seem to have been produced
by relatively unspecialized ancestral lines.'^

* If Garstang’s suggestion be true (see p. 532 ) that clandestine evolution
has enabled new brge-scajc radiations to start by utilizing a larval organization
and driving the adult organization off the stage, we have here an apparent

EVOLUTIONARY- PROGRESS 563

Looked at froiii a sliglitiy different angle, we may say that
progress must in part. at least be defined on the basis of final
results. These- results have consisted iii the historical fact of a
succession of dominant groups. And the cliief characteristic
which analysis reveals as having contributed to the rise of any.
one of these groups is an improvement tliat is not one-sided but'
al-roiind and basic. Temperature-regulation, for instance, is a
property which affects almost every function as well as enabling
its possessors to extend their activities in time and their range
in space. Placental reproduction is not only a greater protection
for the young — -a placental mother, however hard-pressed,
cannot abandon her unborn embryo — ^but diis additional protec-
tion, together witli the later period of maternal care, makes
possible the extension of the plastic period of learning which
then served as the basis for the further continuance of progress.

It might, however, be held that biological inventions such as
the lung and cleidoic shelled egg, which opened the world of
land to the vertebrates, are after all nothing but specializations.
Are they not of the same nature as the wing which unlocked the
kingdom of the air to the birds, or even to the degenerations
and peculiar physiological changes which made it possible for
parasites to enter upon that hitherto inaccessible habitat provided
by the intestines of other animals.^ This is in one sense true; but
in another it is untrue. The bird and the tapeworm, although
they did conquer a n^w section of the environment, in so doing
were as a matter of actual fact cut off from further progress.
Theirs was only a specialization, though a large and notable one.
The conquest of the land, however, not only did not involve
any such limitations, but made demands upon the organism
which could be and in some groups were met by further changes
of a definitely progressive nature,’*^ Temperature-regulation, for

exception. It is only fair to say, however, that this view is still highly speculative,
and that in any case we would presume that a relatively unspecialized larval
type would have served as the new starting-point.

* Moricy Roberts (1920, 1930, etc.) gives numerous interesting examples in
which new and in a sense abnormal demands upon organisms result eventually
in adjustments which are more or less adaptive in relation to the new situation.
Unfortunately he postulates a lamarckian transmission of modifications which
vitiates or obscures much of his evolutionary discussion.

564 EVOX-UTION: THE MODERN SYNTHESIS

instance, cooid never have arisen through natural selection except
in an environment with rapidly-changing temperatures: in tbe
less changeable waters of the sea the premium upon it would
not be high enough * The same is true for curythermy (p- 444 )-
Of course a progressive advance may eventually come to a
dead eni as has happened with the insects, when all the biolo^cai
possibilities inherent in the type of org^tion have bem
exploited. From one point of view it might be permissible to
cJ such a trend a long-range specialization; but n would appe^
more reasonable to style it a form of progress, albeit one wfoch
is destined eventually to be arrested. It is limited as opposed to

unlimited progress. . ,

A word is needed here on the restricted nature of biologic^
progress. We have seen that evolution may involve downward
or lateral trends, in the shape of degeneration or cermm forms of
specialization, and may also leave certam types stable. Further,
lower types may persist alongside higher, even when the lower
are representatives of a oncc-dominant group that includes the
higher types. From this, it will first be seen, as we aheady men-
tioned, that progress is not compulsory and umversal; and
secondly that it will not he so marked in regard to the average
of biological efficiency as to its upper limit. Progress, m other
words, can most readily be studied by examinmg the upper
levels of biological efficiency (as determined by our catena of
control and independence) attained by life at successive penods

of its evolution. ... i

For this, duriotg the earEer part of Efe s history, we most rely

upon the indirect evidence of phylogeny, drawn firom com-
parative morphology, physiology, and embryology, vMe for
the last thousand million years this is further illummated by the

Hght of paleontology, with its dirert evidence of fossils.

We have thus arrived at a definition of evolutionary progress
as consisting in araising of the upper level of biological efficiency,
this being defined as increased control over and independence of

* Once eYobed on land* however, it proved its value even in sea, as
evidenced hj the sucx^ess of the Cetacea and other secondary aquatics among
'imiimials.' (see p. 452).

iiVOLUTIONAKY 1 'ROGRF.SS 565

the environment* As an alternative wc might define it as a
raising of the upper level of all-round functional efficiency and
of harmony of internal adjustment. |

This brings us to a further objection which is often raised to
the idea of progress, namely, tiiat it is a mere anthropomorphism.
This view asserts that wc judge animals as higher or lower by
tlieir greater or lesser resemblance to ourselves and diat we give
the name of progress to the evolutionary trend wliich happens
to have culminated in ourselves. If we were ants, die objectors
continue, we should regard insects as die highest group and
resemblance to ants as the essential basis of a “high” organism :
while if we were eagles our criterion of progress would be an
avian one.

Even Haldane (1932a, p. 153) has adopted diis view. He writes,
“I have been using such words as ‘progress’, ‘advance, and
‘degeneration, as I think one must in such a discussion, but I
am well aware that such terminology represents rather a tendency
of man to pat himself on the back than any clear scientific diink-
ing. . . . Man of to-day is probably an extremely primitive and
imperfect type of rational being. He is a worse animal than the
monkey. ... We must remember that when we speak of progress
in Evolution we are already leaving the relatively firm ground
of scientific objectivity for die sliifting morass of human values.”

niis I would deny. Haldane has neglected to observe that
man possesses greater power of control over nature, and lives
in greater independence of his environment than any monkey.
Tlie use of an inductive method of approach removes all force
from such objections. The definitions of progress that wc were
able to name as a result of a survey of evolutionary facts, though
admittedly very general, arc not subjective but objective in their
character. That the idea of progress is not an anthropomorphism
can immediately be seen if wc consider what views would be
taken by a philosophic tapeworm or jellyfish. Granted that such
organisms could reason, they would have to admit diat diey

* Herbert Spciiccr recognized the importance of increased independence as
a criterion of evolutionary advance: see references in Needham (i937)-

■}■ Sec also R. W. Gerard (1940), “OrgaiHsm, Society, and Science”, 6Vj. M-?.,

566 evolution: THE MODEUN SYNTHESIS

were neither dominant types, nor endowed with any potentiality
of further advance, but that one was a degenerate blind alley,
the other a specialization of a primitive type long left behind
by more successful forms of life. And the same would be equally
true, though not so strikingly obvious, of ant or eagle. Man is
the latest dominant type to be evolved, and this being so, we arc
justified in calling the trends which have led to his development
progressive. We must, however, of course beware of subjectivism
and of reading human values into earlier stages of evolutionary
progress. Human values are doubtless essential criteria for the
steps of any future progress : but only biological values can have
been operative before man appeared.

The value of such a broad biological definition of progress
may be illustrated by reference to a recent definition of human
progress by Professor Gordon Childe (1936). Professor Childe,
too, is seeking for an objective criterion for progress; but the
criterion he adopts is increase of numbers. Quite apart from the
logical difficulty that increase in population must, on a finite
earth, eventually approach a limit, it is clear that this criterion
is at once invalidated by the fiicts of general biology.

There are many more of various common plankton organisms
than of men or of any bird or mammal. There are in all probability
many more houseflies than human beings, more bacteria, even
of a single species, than of any metazoan. If we apply our criterion
of increased control and independence, we see that it would be
theoretically quite possible (though difficult with our present
t5q>e of economy) to obtain progressive changes in human
civilization with an accompanying decline in population.

Here let me interject a further word concerning objective and
subjective criteria for progress. As regards human progress, it
is clear that subjective criteria cannot and should not be neglected ;
human values and feelings must be taken into account in deciding
on the future aims for advance. But in comparing human with
pre-human progress, we must clearly stick to objective standards.
I would thus like to make a distinction between biological or
evolutionary progress and human progress. The former is a
biological term with an olyective basis: it includes one aspect of

EVOLUTIONARY PROGRESS ^67

human progress. Human progress, on the other hand, has con-
notations of value as well as of efficiency, subjective as well as
objective criteria.*

Returning to biology, we may sum up as follows. Progress is
all-round biological improvement. Specialization is one-sided
biological improvement; it always involves the sacrificing of
certain organs or functions for the greater efficiency of others.
It is the failure to distinguish between these two types of evo-
lutionary process that vitiates the generalizations of many
biologists (e.g. Hawkins, 1936).

Degeneration is a form of specialization in which the majority
of the somatic organs are sacrificed for greater efficiency in
adaptation to a sedentary or a parasitic life. Locomotor organs
disappear, sensory and nervous systems are much reduced, and
in parasites the digestive system may be abolished. Reproductive
mechanisms, however, may be inordinately specialized, as in
certain parasites.

Besides these types of evolutionary process, we may have
stability, as in the lamp-shell Lingula, or in ants during most of
the Cenozoic epoch. Stable types are presumably either extremely
well-adapted to a permanent biological niche or have reached
the limit of specialization or of progress possible to them.

Finally, we may have tlie type of evolutionary trend best
known among the Ammonites, of increasing complication
followed by simplification. This we have already discussed in
our section on orthogenesis.

A possible method of evolutionary escape from speciahzation
is afforded by changes in rate of sexual maturity relative to
general development, leading to neoteny or foetalization, as

* On the other hand, to confhie the term progress entirely to human afiaiis,

and to contrast it with evolution in pre-human history, as docs Marett (i933» I939)i.
is to restrict the meaning of progress unduly, while distorting that of evolution.
On three successive pages Marett describes or defines progress in three different
ways: (i) the moral of human history and pre-history would seem to be that
“progress in the direction of the spiritual is implicit in normal human endeavour*’.
( 2 ) Progress in spirituality in the future “may be conceived in terms of the
greatest self-realization of the greatest niunber”. ( 3 ) “Real progress is progress
in charity.” It should be clear how important it is to give greater universality
and concreteness to the idea of progress by considering human progress as a
special case of biological progress.

5fi8 evolution; the modern synthesis

discussed in a previous chapter (pp. 5^6 fF-, 55S)-

aboHsh a specialized adult phase and give die opportunity for ic

progressive evolution of a new generalized type ..

As revealed in die succession of steps that have led o
dominant types, progress has taken diverse forms. At one stage
the combinadon of cells to form a niulticeliular individu 1,
another the evolution of a head; later the devc opment o u g ,

stiU later ofwarm blood, and fkaUy the enhancement of mtcl-

Ugcncc by speech. But all, though in cunomly different ways,
hzve enhanced the organism’s capacities for control and for
independence; and each has justified itself not oidy m immediate
results but in die later steps which it made possible.

We have now dealt with the fact of evoludonary progress,
and with the philosophical and biological difficulties m^erent
in the concept. What of its mechamsm? It should be clear that
if natural selecdon can account for adaptation and for long-range
trends of specialization, it can account for progress too. ro-
ercssive changes have obviously given their owners advantages
wliich have enabled them to become dominant. Sometimes it
may have needed a climatic revolution to give the progressive
change full play, as seems to have been d^ case at die end of the
Cretaceous with the mammal-reptile differential of advantage,
but when it came, the advantage had very large results— whole-
sale extinctions on die one hand, wholesale radiation of new
tvpes on the other. It seems to be a general characteristic of
evolution diat in each epoch a minority of stocks give rise to the
majority in the next phase, while conversely die majority of the
rest become extinguished or arc reduced in numbers.

There is no more need to postulate an elan vital ox a guidmg
purpose to account for evolutionary progress than to accouiit
for adaptation, for degeneration or any other form of speciali-
zation.* . ,

One point is of importance. Although wc can quite correctly

speak of evolutionary progress as a biological fact, this progress

* A small niinoritv of biologists, such as Broom (i933).
to invoke “spiritual agencies” to account tor progressive ^

immbcr is decreasing as the iniplicatioiis of modem selection tncT>! ics * h * I

EVOLUariONARY PROGRESS 5<59

is of a particular and limited nature. It is, as we have seen, an
empirical fact that evolutionary progress can only be measured
by the upper level reached : for the lower levels are also retained.
This has on numerous occasions been used as an argument against
the existence of anything which can properly be called progress;
but its employment in this connection is fallacious. It is on a par
with saying that the invention of the automobile does not repre-
sent an advance, because horse-drawn vehicles remain more
convenient for certain purposes, or pack animals for certain
localities. A progressive step in evolution will normally and
probably invariably bring about the extermination of some types
at a lower level; but the variety of environments and of the
available modes of filling them is such that it is extremely
unlikely to exterminate them all. The fact tliat protozoa should
be able to exist side by side with metazoa, or a considerable
army of the “defeated” group of reptiles together with their
mammalian “conquerors”, is not in any way surprising on
selectionist principles : it is to be expected.

4. THE PAST COURSE OF EVOLUTIONARY PROGRESS

One somewhat curious fact emerges from a survey of biological
progress as culminating for the evolutionary moment in the
dominance of Homo sapiens. It could apparently have pursued no
other general course than that which it has historically followed:
or, if it be impossible to uphold such a sweeping and universal
negative, we may at least say that among the actual inhabitants
of the earth, past and present, no other hues could have been
taken which would have produced speech and conceptual
thought, the features that form the basis for man^s biological
dominance.*

Multicellular organization was necessary to achieve the basis
for adequate size: without triploblastic development and a blood-
system, elaborate organization and further size would have been
impossible. Among the coelomates, only the vertebrates were
eligible as agents for unlimited progress, for only they were able
* So far as I am aware, this was first emphasized by Huxley, 2936-

570 evolution; THE MODERN SYNTHESIS

to achieve the combination of active efficiency, size, and terrestml
existence on which the later stages of progress were
based. Only in the water have the molluscs achieved y g
advance. The arthropods are not only hampered by their necessity
for moulting; but their land representatives, as was first pomtc
out by Krogh, are restricted by their tracheal respiration to very
smaU^size. They arc therefore also restricted to cold-bloodedness
and to a rehance on instinctive behaviour (see
WeUs, Huxley and Wells, 1930, Book 5, chap. 5 , § 7 ). Lungs
were one needfiil precursor of inteUigence Warm blood was
another since only with a constant internal environment could
*e brain achieve Lbility and regnWty for te
This limits us to birds and mammals as bearers of the torch of
progress. But birds were ruled out by their deprivmg themselves
of mitential hands in favour of actual wmgs, and perhaps also by
the restriction of their size made necessary m the interests of flight.

Remain the mammals. During the Tertiary epoch most
mammalian lines cut themselves off from the possibflity of
ultimate progress by concentrating on immediate specialization.
A horse or a lion is armoured against progress by the very
efficiency of its Hmbs and teeth and sense of smep. it is a hinited
piece of organic machinery. As Elliot Smith has so My set
forth, the penultimate steps in the development of our human
intelligence could never have been taken except m arboreal
ancestors, in whom the forelimb could be converted mto a hand,
and sight inevitably became the dominant sense in place of sme^
But, for the ultimate step, it was necessary for the anthrogid
to descend from the trees before he could become man. This
meant the final liberation of the hand, and also placed the evolving
creature in a more varied environment, in which a iig^ er
premium was placed upon intelligence. Further, the foctalization
necessary for a prolonged period of learning could only have
occurred in a monotocous species (pp. 525. 555 ; Haldane, 1932U,
p. 124; Spence and Yerkes, i 937 )- Weidcnrcich (1941) mam-
tains that the attainment of the erect posture was a necessary

prerequisite for the final stages in human cerebral evolution.

The last step yet taken in evolutionary progress, and the only

EVOLUTIONARY PROGRESS

571

one to hold out the promise of unlimited (or indeed of any
further) progress in the evolutionary future, is the degree of
intelligence which involves true speech and conceptual thought:
and it is found exclusively in man. This, however, could only
arise in a monotocous mammal of terrestrial habit, but arboreal
for most of its mammalian ancestry. All other known groups of
animals, except the ancestral line of this kind of mammal, are
ruled out. Conceptual thought is not merely found e-^clusively
in man : it could not have been evolved on earth except in man.

Evolution is thus seen as a series of blind alleys. Some are
extremely short — those leading to new genera and species that
either remain stable or become extinct. Others are longer — the
lines of adaptive radiation within a group such as a class or sub-
class, which run for tens of millions of years before coming up
against their terminal blank waU. Others are stiU longer — the
lines that have in the past led to the development of the major
phyla and their highest representatives; their course is to be
reckoned not in tens but in hundreds of mil li ons of years. But
all in the long run have terminated blindly. That of the echino-
derms, for instance, reached its dimax before the end of the
Mesozoic. For the arthropods, represented by their highest
group, the insects, the foil stop seems to have come in the early
Cenozoic: even the ants and bees have made no advance since
the Oligocene. For the birds, the Miocene marked tlie end; for
the mammals, the Pliocene.

Only along one single line is progress and its future possibility
being continued — the line of man. If man were wiped out, it
is in the highest degree improbable that the step to conceptual
thought would again be taken, even by his nearest kin. In the
ten or twenty million years since his ancestral stock branched off
from the rest of the anthropoids, these relatives of his have been
forced into their own lines of specialization, and have quite left
behind them that more generalized stage from which a conscious
thinking creature could develop. Although the reversibility of
evolution is not an impossibility per se, it is probably an actual
impossibility in a world of competing types. Man might con-
ceivably cause the capacity for speech and thought to develop by

573 evolution: THE MODERN SYNTHESIS

loDg and intensive selection in the progeny of topanzees or
goraias; but Nature, it seems certain, could never do so

One of the concomitants of organic progress has ,

progressive cutting down of the possible ^

progress, until now, after a thousand or fifteen hun Jed mjion
years of evolution, progress hangs on but a single
Lead is the>unun germ-plasm. As Vilhers J Hsle-A^
wrote in VEve Future, “L’Homme . . . seul, dans lumvers,

n’est pas fini.”

5 . PROGRESS IN THE EVOLUTIONARY FUTURE

What of the future ? In the past, every major step in evolutionary
progress has been followed by an outburst of change. For one
thing the familiar possibilities of adaptive ra Jatton may
exploited anew by a number of fresh types which dommate or
extinguish the older dispensation by the aid of the new piece of
orgaiTic machinery which they possess. Or, when the progressive
step has opened up new environmental realms, as was the ca^
widi lungs and the shelled egg, these are conquered and people J
or thefimdamental progressive mechanismmay itselfbe improved,
as was the case with temperature-regulation or the pre-natal
care of the young in mammals. ^

Conscious and conceptual thought is the latest step m iifes
progress. It is, in the perspective of evolution, a very rec^t one,
having been taken perhaps only one or two and certjily Im
fhan ten milEon years ago. Although already it hss been me
cause of many and radical changes, its main effects are mdubitably
still to come. What will they be? Prophetic phantasy is a danger-
ous pastime for a scientist, and I do not propo^ to indulge it
here. But at least we can exclude certain possibilities. Man, we
can be certain, is not within any near future destined to break
up into separate radiating lines. For the first time in evolution, a
new major step in biological pro^«s will produce but a single
species. The generic variety achieved elsewhere by radiating
divergence will with us depend primarily upon crossing and

recombination (see Huxley, 1940).

EVOLUTIONARY PROGRESS 573

We can also set limits to the extension of his range. For the
planet which he inhabits is limited, and adventures to other
planets or other stars are possibilities for the remote foture only.

During historic times, all or almost all of the increase in man’s
control over nature have been non-^enetic, owing to his exploita-
tion of his biologically unique capacity for tradition, whereby
he is provided with a modificational substitute for genetic change.
The realization of the possibilities thus available will continue
to play a major part in human evolution for a very long period,
and may contribute largely to human progress.

More basic, however, though much slower in operation, are
changes in the genetic constitution of the species, and it is evident
that the main part of any large genetic change in the biologically
near future must then be sought in the improvement of the
fundamental basu of human dominance — the feeling, thinking
brain, and the most important aspect of such advance will be
increased intelligence, which, as A. Huxley (i937. p- 265) has
stressed, implies greater disinterestedness and fuller control of
emotional impulse.*

First, let us remind ourselves that, as we have already set forth
(p. 482), we -with ouf human type of society must give up any
hope of developing such altruistic instincts as those of the social
insects. It would be more correct to say that this is impossible
so long as our species continue in its present reproductive habits.
If we were to adopt the system advocated by Muller (1936) and
Brewer (1937), of separating the two functions of sex — ^love
and reproduction — and using the gametes of a few highly
endowed males to sire all the next generation, or if we could
discover how to implement the suggestion of Haldane in his
Daedalus and reproduce our species solely from selected germinal
tissue-cultures, then all kinds of new possibilities would emerge.
True castes might be developed, and some at least of them
might be endowed with altruistic and communal impulses.
In any case, as A. Huxley (i937) points out in an interesting

* Of course great increases in man’s control over and mdep^dence of to
enviromnent may be produced by the better utilization of his existing capacid<^
(see e.g. G. H. Thomson, I93<S); but these represent modifications, not genetic
Ganges.

574 evolution: THE MODERN SYNTHESIS ^

discussion, progress (or, I would prefer to say,_future human
progress) is dependent on an increase of intraspecdic co-operation
until it preponderates over intraspecific competition. . ,

Meanwhile there are many obvious ways in winch the brain s
level of performance could be genetically raised— m acuteness
of perception, memory, syntlietic grasp and intuition, analytic
capacity, mental energy, aeative power, balance, and judgment.

If for all these attributes of mind the average of our popidation
could be raised to the level now attained by the b^t endowed
ten-thousandth or even thousandth, that alone would be of tar-
reaching evolutionary significance. Nor is there any reason to
suppose that such quantitative increase could not be pushed

beyond its present upper limits. r u - u

Further, there are other faculties, the bare existence of which
is as yet scarcely estabUshed: and these too might be developed
until they were as commonly distributed as, say, musical or
mathematical gifts are to-day. I refer to telepathy and other
extra-sensory activities of mind, which the painstaking wor o
Rhine (1935), Tyrrell (1935). others is now forcing upon the
scientific world as a subject demanding close analysis. ^

If this were so, it would be in a sense only a continuation of a
process that has already been at work— the utilization by man for
his own ends of hitherto useless by-products of his mental
constitution. The earlier members of the Hominidae can have
had little use for the higher ranges of aesthetic creation or appre-
ciation, for mathematics or pure intellectual construction. Yet
to-day tliese play a large part in human existence, and have come
to possess important practical consequences as well as value in
and for themselves. The development of telepathic knowledge
or feeling, if it really exists, would have equaUy important

consequences, practical as well as intrinsic.

In any case, one important point should be borne in mind.
After most of the major progressive steps taken by life in the
past, the progressive stock has found itself handicapped by
characteristics developed in earlier phases, and has been forced
to modify or abandon these to realize the full possibilities of the
new phase {see M. Roberts, 1920, 1930, for various examples of

EVOLUTIONARY PROGRESS 575

forced adjustment to new conditions, but with the caveat that
some are highly spec'jlative, and that aU are presented in a
lamarckian frame of reference which often obscures their true
significance). This evolutionary fact is perhaps most obvious in
relation to the vertebrates’ emergence from water on to land.
But it applies in other cases too. The homothermy of mammals
the Strapping of scales and the substitution of hair;
man’s erect posture brought with it a number of anatomical
inconveniences. But man’s step to conscious thought is perhaps
more radical in this respect than any other.

By means of this new . gift, man has discovered how to grow
food instead of hunting it, and to substitute extraneous sources
of power for that derived from his own muscles. And for the
satisfaction of a few instincts, he has been able to substitute new
and more complex satisfactions, in the realm of morahty, pure
intellect, aesthetics, and creative activity. ^

The problem immediately poses itself whether man s muscular
power and urges to hunting prowess may not often be a handicap
to his new modes of control over his environment, and whether
some of his inherited impulses and his simpler irrationd satis-
factions may not stand in the way of higher values and fuller
enjoyment. The poet spoke of letting ape and tiger die. To tins
pair, the cynic later added the donkey, as more pervasive and in
the long run more dangerous. The evolutionary biologist is
tempted to ask whether the aim should not be to let the mammal
die within us, so as the more effectually to permit the mm to hve.

Here the problem of values must be faced. Man differs from
any previous dominant type in that he can consciously orm ate
values. And the realization of these in relation to the priority
determined by whatever scale of values is adopted, must accord-
ingly be added to the criteria of biological progress, once advance
has reached the human level. Furthermore, the introduction of
such criteria based upon values, in addition to the
more objective criteria of increasing control and mdependence
which sufficed for pre-human evolution, alters the dirertion ot
progress. It might perhaps be preferable to say that it ten t le
level on which progress occurs. True human progress consists in

576 evolution: THE MODERN SYNTHESIS

increases of aesthetic, intellectual, and spiritual experience and

“ increase of conttol and of independence is necessary

for the increase of thc^e spiritual satisfactions; but e more or

less measurable and objective control over f ^
external environment are now merely subsidiary mechanism
scrvii^ as the material basis for the ^u^ian type o^ ’

and the really si^cant control .

man’s mental states— his control of ideas to give “ .

satisfaction, of form and colour or of somd
satisfkction. his independence of inessential stimuh and ideas to
give the satisfaction of mystic detachment and inner ecsmy.

® The ordinary man. or at least the ordinal poet philosopher
and theologian, is always asking himself what is the purpose of
human life, and is anxious to discover some extraneous purpose
to which he and humanity may conform. Some find such a
purpose exhibited directly in revealed religion; oAers ^
that they can uncover it from the facts of nature. One of the
commonest methods of this form of natural religion is to pomt
to evolution as manifesting such a purpose. The history ot hie,
it is asserted, manifests guidance on the part of some exter^
power; and the usual deduction k that we can safely trust that
samp power for further guidance in the future.

I believe this reasoning to be wholly false. The purpose mam-
fested in evolution, whether in adaptation, specialization, or
biological progress, is only an apparent purpose (p. 412). it « as
much a product of blind forces as is the falling of a stone to earth
or the ebb and flow of the tides. It is we who have read purpose
into evolution, as earlier men projected will and emotion mto
hiorganic phenomena like storm or earthquake. If we wish to
work towards a purpose for the future of man, we mu&t formulate
that purpose ourselves. Purposes in life are made, not found-

But if we cannot discover a purpose in evolution, we cm
discern a direction— the line of evolutionary progress. And this
past direction can serve as a guide in formulating our purpose
for the future. Increase of control, increase of independence,
increase of internal co-ordination; increase of knowledge, o

EVOIUXIONAHY PROGRESS

577

means for co-ordinating knowledge, of elaborateness and intensity
of feeling— those are trends of the most general order. If we do
not continue them in the future, we cannot hope that we are
in the main line of evolutionary progress any more than could
a sea-urcliin or a tapeworm.

As further advice to be gleaned from evolution there is the
fact we have just discussed, that each major step in progress
necessitates scrapping some of the achievements of previous
advances. But this warning remains as general as the positive
guidance. The precise formulation of human purpose cannot be
decided on the basis of the past. Each step in evolutionary progress
has brought new problems, which have had to be solved on their
own merits; and with the new predominance of mind that has
come with man, life finds its new problems even more unfamiliar
than usual. This last step marks a critical point in evolution, and
has brought Hfe into situations that differ in quahty from those
to which it was earHer accustomed.

The future of progressive evolution is the future of man. The
future of man, if it is to be progress and not merely a standstill
or a degeneration, must be guided by a deliberate purpose. And
this human purpose can only be formulated in terms of the new
attributes achieved by Hfe in becoming human. Man, as wc have
stressed, is in many respects unique among animals;* his purpose
must take account of his unique features as well as of those he
shares with other life.

Human purpose and the progress based upon it must accord-
ingly be formulated in terms of human values; but it must also
take account of human needs and limitations, whether these be
of a biological order, such as our dietary requirements or our
mode of reproduction, or of a human order, such as our intel-
lectual limitations or our inevitable subjection to emotional
conflict.

Obviously the formulation of an agreed purpose for n}an as
a whole will not be easy. There have been many attempts already.
To-day we are experiencing the struggle between two opposed

* For a full analysis of the biological peculiarities of our species see Huxley,

578 evolution: THE MODERN synthesis

ideals-that of the subordination of the indiiddual to the commu-
nity. and that of his intrinsic supenonty. Another struggle soli
in progress is between the idea of a purpose directed to a foture
life in a supernatural world, and one directed to projr^s in tim
existing world. Until such major conflicts are ^

can have no single major purpose and progress cm ^e but fi&l
and slow. Before progress can begin to be rapid, mm must
cease being afraid of his uniqueness, and must not conmue to
put off the responsibiHties that are really his on to
of mythical gods or metaphysical absolutes (see Everett. 93 )•

But let us not forget that it is possible for progress to be
achieved. After the disillusionment of the early twentieth century
it has become as fasHonable to deny the existence of progress
and to brand the idea of it as a human illusion, as it was fashion-
able in the optimism of the nineteenth cenmry to proclaim not
only » odstmce bnt to mevitabiUty, The on* o betwe<n the
two eitoemes. Ptogtess is a majot &a of past evolutiOT, but it
is limited to a few selected stocks. It may continue m the fiimre,
but it is not inevitable; man, by now become the trustee of
evolution, must work and plan if he is to achieve ftirther progress

for himself and so for life.

This limited and contingent progress is very different from the
deus ex machim of nineteenth-century thought, and our optimism
may well be tempered by reflection on the difficulties to be
overcome. None the less, the demonstration of the existence of
a general trend which can legitimately be called progress, and
the definition of its limitations, will remain as a fimdamental
contribution of evolutionary biology to human thought.

' * See also Huxley, I943. Evciutionary Ethks (Romanes Lecture) University

Press, Oxford. ADDENDUM I955

For recent dei’clopiiiefits readers are referred to the following works:

paleontology, course of evolution : G. t:. simpson ThcMajor

Ffatures of Evolution.” New York, 1953 . . ,

SPECIATION AND GENETICS: t. dobzhansky^ Genetics Oqgi

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Sg^APMCAL^RUO^^ and GENERAL SYNTHESIS: b. rensch
” toe^PrSaleme der Abstammungslehre.” (and. edition) Stuttgart. 1954-
EVOLUTIONARY ETHICS: and PROGRESS; t. h. and j. s. hux e .

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INDEX

SUBJECTS

Abdomen, abnormal, 63
Abmigration, 238
Accident, role of, in evolution, 58
Acbondroplasia, 553
Activity-range, 238
Adaptation[s],'i22, 153
analysis of, 449
and function, 417 if.

' and selection, 37 if.

: and teleology, 412 :

, : as a; relative concept, 43.8 C
classification of, 419
examples of, 417 ff.
in avian display, 425
in desert animals, 422, 43 3
in instinct, 42S
in parasites, 429
In symbiosis, 429'

, incomplete, 448
induced, 428
internal, 185, 420
interpretation of, 34 ff.

. interspecific, 419 ^
intraspecific, 419

not necessarily beneficial to * the
species, 478 k,
omnipresence of, 412 C
origm of, 412, 457 ff., 466 ff.
passive, 449
perfection of, 447
physiological* 436
■ re^arities of, 430 ff.

. , ;reHct, 477" '

’ ...^.respiratory, 430
types of, 417 ff.

Adaptive radiation, 324, 389
trends in, 486 ff.

Adaptive trends, apparent ortho-
genesis of, 497 ff.
selective determination of, 494 ff.
Adjustment, of new mutations, 124
Advantage, selective, sec Selective
advantage
Agamospecies, 166
Aggregation[s], 479
consequential effects of, 554

Alleles, multiple, 65, 79
AHeffs rule, 213

AHometry, 127, 178 n., 213, 529,

53$ ff.

in anteaters, 537
Aliomorphosis, 529 n.

AHopolypioidy, 45, 340 ff., 451, 478
functional, 143
AEotetraploidy, 87, 141
Altruism, 482, 573
Ancestral inheritance, law of, 152
Aneuploidy, 88, 349, 362
Anisopolyploidy, 140
Antilles, amphibia of, 234
Apomict strains, 408
Apomixis, 45

and “fossil” adaptations, 477
consequences of, 150
in Crepis, 374
Arrhostia^ 505

Artenkreis, 170, 179, I97» 4^4* 4^7

Atavism, 20
Autogamy, 108

Autopolypioidy, 45, 128, 334 ff., 45^
functional, 144 ff.
reduction of plasticity in, 143
Autotctraploidy, 87, 141
Autotomy, 418

Baikal, fauna of, 181, 493
Balance, ecological, loi ff.
genetical, 97
genic, 64 ff.
of nature, no
selective, 97 ff.

Balanced lethals, 90, 329, 3S1
Barriers, geographical, 228
to crossing, 287, 298, 369, 37^. 3^5
Bergmann’s rule, 212, 283
Biogeography, 31

Biological progress, see Progress, bio-
logical

Biological races, 166
Biometry, 24, 26
Biotype, 166

6 l 6 evolution:/ the' MOBERN;^

Blood-gfowps, S3 .

Bufieiing, of mutations, 6S
ButterEies, colour-patterns, 550

Calcicole and calcifuge, plants, 273 ff.
California, avifauna, 188
Cave fauna, blind, 453
Cel-tlieory, 170
Centromere, 136
and inertness, 139
Certation, 481

Character-gradients, 190, 193, 206 C,
547; see Clines
Characters, allaesthetic, 289
and genes, 19

correlated, 53, 63, 188 ff,, 206, 430,

.509,533

mendelian, 62
wild type, stability of, 73
Cbiasma-frequency, 137
Chiasmata, function of, 136
Chromosomal isolation, 332
Chromosome-races, 408
Chromosomes, as super-molecules, 49
giant, 331
inert, 339 n.
nature of, 86

Classification, natural, 410
Cieistogamy, 107
Climatypes, 275

Clines, 130, 160, 180, 190, 193, 203,
206 ff, 261, 380, 407, 408, 430 ff.
altitudinal, 223

difference from subspecies, 226
in degree of interspecific crossing,
355

intergroup, 211
mapping of, 225
nomenclature of, 227
parallelism of subspecific, 215
polymorph ratio, 103. 217, 221,
terminology of, 226
Clones, 382, 408
Clutch-me, clines in, 215
Coloration, and climate, 213
concealing and revealing, 414, 483
Colour-variations, parallel, 321
Colour-vision, in mammals, 519
Comb-form, in fowls, 20
Commiscuunif 163
Common ancestry, 398
Community, ecological, 169, 209 n.
Competition, and man’s evolution, 485

■ Coiyugation, and unfavouraMe condi-

tions, 84

Conservation, 201
Corn Laws, and plant breeding, 16
Correlated characters, see Characters,
correlated
Crest, in fowl, 73
Crinkled dwarf, in cotton, 77
Crosses, inter-specific, 114, 1 17, 146 ff.
Crossing, prevention of, in nature, 287
barriers against, 289
Crossing-over, 331
and centromere, 139
and genes, 49

and unfavourable conditions, 85

Darwinian epoch, 391
Darwinism, 13 ff., 475
deductive element in, 21
eclipse of, 22 ff.
revival of, 26

Dauermodifikatiomnt 300, 459
Deficiency, 89

Deforestation, evolutionary effects of,
I47»258

Degeneration, 42, 455, 476, 567
Deme, definition of, 203
Development, differential, conse-
quences of, 525 ff.

Differentiation, see Divergence
and time, 205
biological, 295 ff., 302
geographical, 259 ff, 263 ff.
local, 263 ff.

physiological, 295 ff , 308 ff
reproductive, 308 ff.

I Dimorphism, see Polymorphism
I geographical, 184
sexual, 222

|witch-controI of, 102
Diploidy, evolution of, 134
Discontinuity, biological, 165, 169
partial, 209, 260
Disharmony, reproductive, 186
Display, 35, 4^5, 426, 5:^5
Distinctiveness, biological value of,

■ . ■ 289

Divergence, accidental, see Drift
biological, 295 ff.

ecobiotic, 228, 271 n., 274, 280, 295
ecodimatic, 228, 266 ff.
ecogeographical, 266 ff.
ecological, 228, 265 ff.
ecotopic, 228, 268 n., 269, 272, 323

I N D E X—S U B J FX' T S

617

Divergence (amtiinied)

. gcncdc, 32K ft',
oiorphoiogical, 296
phydologica!, 295, 308 if.
with low competition, 323 ft'.
Domestic breeds, recessive characters
ill, 81

Dominance, and position-eifect, 86
degree of, 72
evolution of, 75 C
in multiple aEelic series, 79
reversal of, 543
Dominant groups, 560
Dominants, selection of, 57
Drift, sH, loi, 155, 193, 194, ^ 99 » 204,
242, 259, 265, 362, 380, 384
Ductless glands, morphogenetic cftect
of, 553

Duplication, 89

Dwarf types, on islands, 12 1

Ecobiotic divergence, 228, 271 n.,
274, 280. 295

Ecoclimatic divergence, 228, 266 ft".
Ecocline, 223, 275
Ecological divergence, 228, 265 C
types of, 228
Ecology, 129

Ecotopic divergence, 228, 268 n.,
269 ff, 272, 333
Ecotype, 275, 407, 437
climatic, 275
multiple, 277
seasonal, 276
Efficiency, organic, 489
Elan vital, 458, 568
Elimination, selective, 225
Equilibrium-position, in rate-charac*
ters, 74

Eumelanins, 213
Euryhalinc forms, 444
Euryplastic forms, 444, 519
Eury thermic forms, 444
Evolution, 13, 68
and speciadon, 387 ffi
clandestine, 532, 543,
comparative, 128, 130
consequential, 525 ff, 543 ffi
convergent-divergent, 353
direcdonai, 39
discontinuous, 27
heterogeneity of, 29 ff.
irreversibility of, 501

Evolution (contim4ed)
non-adaptivc, 59
orthosclcctive, 523
parallel, 409, 488, 515
parallelism in, 487
rate of, 56
reverse, 502
reticulate, 167, 351 ff.
three aspects of, 40
Evolutionary progress. 42, 556 If.
definidon of, 559 ff.
mechanism of, 562
nature of, 562
past course of, 569 ff.

Evolutionary trends, 42, 486 if,
consequential, 543 ff.

Existence, Struggle for, see Struggle
for Existence
Exogamy, 108

Expression, genic, akcratioii of, 68 C
of characters, 20

of genes in different environments,
63ffi

single-dose, 73 n.

Expressivity, heterozygous, 73 n.
Extinction, 20 r, 484, 505 if.

Eyeless, change in effects of, 69, 122

Fertilization, struggle for, 481
Fin-rays, dines in, 215
Fluctuations, in population-size, 33, 61
Foetalization, 526, 532, 543, 555, 570
Formenkreis, 163 n.

Fossils, trends in evolution, 32, 400
Function, concept of, 420

Galapagos, birds of, adaptive radiation,

325

evolution on, 44
fauna, 183, 242
ground-finches of, 290
tameness of birds, 310
Gene, genetic and somatic environ-
' ment of, 65

Gene-combinations, selection of, 57
Gene-complex, 64 C
selection of, 122
stabilised, 1 30, 208, 229
Gene-frcquency, and population-size,
S9

znd selection, 60
Gene-mutation, 87

6i8

evolution: the modekn synthesis

Genera, in taxonomy, 404
horizontal, in fossils, 400
Genes, 47 '

. and characters, 62 C
■ ■ ■ '. evolution of, 132 ff.

size and number of, 50 ■

Genetic divergence, 328 C
Genetic systems, evolution of, 131 ff.
Genocline, 253
Genbme-mutation, 87
Geographical differentiation, 263 ff.
Geographical differentiation, princi-
ples of, 259 ff.

Geographic^ rules, 21s, 225, 516
Gloger’s rule, 21 3 , 257, 43 3
Gradients, environmental, see Clines
in development, 547 ff.

Grafting, intcfrspecific, 3^9*

Group disdnaiveness, 289
Gynodioecism, 107, 140

Habitat-selection, 469
Haemophilia, 55, 59
Haldane's rule, 146
Haploidy, as normal condition, 335
male, 149
Hawaii, fauna, 183
land-snails of, 234
Heart-weight, relative, 215
Heredity, in Darwin's day, 16
Hernia, cerebral, in fowl, 73
Heterauxesis, 529 n.

Heterosis, 139
Heterostyly, 107, 108
Homology, 319 ff., 510 ff, 544
Darwinian concept of, 395
Hooded pattern, rats, 54
Horses, evolution of, 32
Hybrid sterility, 361
Hybrid swarms, I47» 343, 353, 355
Hybridity, structural, 139
Hybridization, 451
Hybrids, nomenclature of, 408
Hyperpituitarism, in dog, 71
Hypertely, 483, 485

Ice-age, and range-changes, 243, 445
and speciation, 146, 165 n., 196,
269 ff, 338

evolutionary effects of, 146
Inactivation of genes, 82
Inactivation theory, recessivencss, 80

Inbreeding, 107, 140
Individuality, 170
Inertness, in Y -chromosome, 138
Infertility, as criterion of species, 164
Inheritance, blending, 25 ff, 55, 151
particulate, 55, 132, 390
Interbreeding, zones of, 255
Interchange, segmental, 90, 198, 329
Intergradation, as specific criterion, 163
deSnition of, 160
zone of, 209, 21 1
Intersexuality, 64
Intestine-size, 215
Invasion, double, 255, 284
Inversion, 91, 93» 330, 362
and crossing over, 331
Islands, oceanic, 230, 324 ff
Isolation, 129, 155» 3<So, 383 n.
ecological, 279, 291
geographical, 175
in domestic breeds, 186
psychosexual, 291
Isophenes, 104, 226
Isopolyploidy, 140

Lakes, African, fauna of, 18 1, 324, 492
Lamarckism, 31, 38, 123, 303 n., 457 ff
experimental testing of, 303
inadequacy of, 457 ff
Lethals, and dimorphism, 97
Lethals, balanced, 90, 329, 381
Life-cycle, and evolution, 137 ff
Lineages, 400, 409
Linkage, and polymorphism, 99

Macro-evolution, 456
Man, evolution in, 399
Mapping, as taxonomic method, 225
Meiosis, evolution of, 133
Meiotic, stage of genetic system, 133
Meiotic system, evolution of, 136 ff.
Melanism, 93 ff.
and climate, 105
dominant, 95
industrial, 94 ff, 470
recessive, 95

Mendelism, and evolution, 26
Metamerism, -compared with poly-
ploidy, 144

Microchromosomes, 366
Micro-evolution, 456
Micro-race, 202, 406
I Microspecies, in Crepis, 374

INDEX— SUBJECTS 619

Orthogenesis, 3i» 3S» 40* 123, 173, 465,
404, 50 r, 504 552

apparent, 484, 497 C
dominant, 510
subsidiary, 5 10
Orthoselection, 500, 523
Ossicles, auditory, 40
Otocephaly, guinea-pigs, 550
Outbreeding, 107, 140
Overlap, morphological, 16011.

Paedogenesis, 533
Paleontology, 30 If., 400
and Lamarckism, 37
Parallel variation, see Variation, parallei
Paramorph, 408

Parasites, reproductive adaptation in,
316,429

Particulate inheritance, 55, 132, 390
Pedigrees, human, 20
Phaeomelaniiis, 213
Phases, 96, 159, 184
Phenocontour, 104
I Phenocopy, 434» 5i2n.

Phcnogenetics, 555
Phylogeny, and taxonomy, 399 if.
Physiology, comparative, 30
Pigmentation, dines 30,213
Piasmageues, 132

Plasticity, 83, 134, 368, 441, 449, 496
and mutation-rate, 137
and recombination, 136 C
and taxonomy, 394
evolutionary, 83 fF.
modificational, 441, 519
of species, 239, 368
of switch type, 74
Pieiotropism, 62
Poikiiothermy, 435
Pollination, sell- and cross-, 108, 417
Polydactyly, 72
Polyisomerism, secondary, 551
Polymorph ratio dines, 217, 221
Polymorphism, 44, 74 n., 96, 159 n.,
189,217,383
environmental, 96
genetic, 96 ff, 1 19
geographical, 184
mimetic, 133

Polyploid complex, in Crepis, 374
Polyploidy, 140 if., 166, 334 if'» 370 i
■'""45X- ^

and colchicine, 346
and taxonomy, 403

Microsubspecics, 201 ff , 406, 408
' Migration, across barriers, 229
and fluctuation, 113
and speciadon, 231, 294, 379
and subspcciatioii, 196, 209
Mimicry, 413, 416, 448, 464, 465.
4 %. 515. 51S
Batesian, 449
Mullerian, 321

polymorphic, in butterflies, 1 01, 122,
159,191,217

Mitotic, stage of genetic system, 13 1
Mobility, and speciadon, 1 55, 239
Modifiers, in evolution, 68
Morphology, comparadve, 22
Mortality, non-sdective, 16
Mutation [s], 17
and evolution, 115 ff.
and recombination, 21
and selection, 47 flf.
direction of, 54
genic, 51

geographical variation in, 199
homologous, 510, 51 1
in bacteria, 1 3 1
in pure lines, 52
induced, 50
interaction of, 121
non-homoiogous, 51 1
of Waagen, 174
parallel, see Variation, parallel
sex-linked, 1 17
siiiall, ti6
systemic, 456
types of, 87 ff
Mutation-rate, 54, 137, 358
and plasticity, 137

Natural Selection, see Selection
and adaptation, 466 ff.
and spedation, 3 84
as originator of adaptations, 473
deduced by Darwin, 14
Neoteny, 527, 532, 543
Nkosi, antelopes of, 236
Nomenclature, of dines, 227
taxonomic, 1 56, 404
Nothodines, 227

Oceanic faunas, 323 ff
Oceanic islands, and flighticssness, 243
Organic Selection, see Selection, Or-
ganic

620 ■ ETOIUTION:::' THB'/MODERN. synthesis

Voljfloidj {continuei)
compared with metamerism, 144
CYolntiomary consequences of, 143 C
' in Crepis, 373
.mPaeGuk^^Zi
■ pliysiological effects, 145
secondary adjustment of effects, 145
Polytocy, 128, 525
Population, and evolution, 60
Population-pressure, 209, 231
Population-size, and spedation, 128,

237

eiectivc, 201 '

Population-structure, and speciation,
262

Populations, size of, 128
small, non-adaptive evolt^tion in, 58
Position-effect, 48, 85 C, 330
and mutation, 92

Preadaptation [s], 52, 198, 315, 327, 419,

449.., ff«

climatic, 445
constitutional, 452
mutational, 450, 454
thermal, 451

Predator-pressure, 18711., 200
Predisposition, 452

Pre^meiotic, stage of genetic system,

132

Pre-mitotic, stage of genetic system, 13 1
Presence and Absence theory, 80
Progress, biological, 489, 556 ff.
biological, restricted nature of, 564
evolutionary, see Evolutionary pro-
gress; 42, 5 5^ if.
human, 566
Pseudo-mutation, 93
Pure lines, in beans, 52
Purpose, in evolution, 576
of human life, 576
Purposiveness, in nature, 412
Races, biological, 166, 297
physiological, 166, 319
Radiation, adaptive, see Adaptive
radiation

Range, changes of, 231, 243 ff., 445
and differentiation, 261
Range-discontinuity, 241
Range-restriction, and competition,
447

Rassenkreis, 163, 170, 179, 218, 242,
354 n.. 374» 403, 4^4
Rate-genes, 508
concept of, 528

Rearrangemeiit[sj, sectional, 89 If., 93,

■ , 139, 333* 361, 457 :

taxonomy and, 365
Recapitulation, 507, 543
Recessive, umversal, 100
Recessiveness, inactivation theory of, 80
Recessives, scarcity of sex-Miiked, 117
selection of, 57
sex-linked, 59, 1 17
Recessivity, degree of, 73 n.
Recognition, 45
characten, 288
Recombination, 51
and mutation, 21
and plasticity, 136 ff.
chromosomal, 133
evolution of, 133
Recombination-index, 137
Refuges, in ice-age, 377
Regeneration, 418

Repeats, evolutionary significance of,
142

of chromosome sections, 89, 142
Repetition, serial, of structures, 551
Replacement, geographical, 174 ff
Reproductive parasites, 3 1<5
Reversion, explanation of, 20
Rhio Linga Archipelago, fauna, 184
Rose-comb, 19
Rule, Alien’s; 213
Bergmann’s, 212, 383
Gloger’s, 213, 257, 433
Haldane’s, 146

Rules, geographical, 211, 225, 257,

283,433*51^

Salinity, and size, 215
Saturation, of gene effects, 82
Scotland, indigenous mammals of, 183
Sectional rearrangement, Sp ff., 139,
333

Segregation, asexual, 334, 352
interspecific, 145

Selection, and periodic fluctuation,

III ff'..

artificial, 482

artificial, and dominance, 81
destructive, 28
. directive, 28

experimental demonstration of, 468
for temperature-resistance, 212
intergroup, 479
interspecific, 128, 478
intrasexual, 35, 481, 525 n., 545, 555

INDEX— SUBJECTS " ■ ' ■ /"■'-'Ml

Seleetion (contimei)
intraspecific, 34, 128* 478, 483, 508
intraspecific, a biological evil, 484
intrauterine, 481, 525
natural, see Natural Selection
not necessarily beneficial, 478 if.
organic, 17, 114, 296, 304, 523
quantitative considerations, 56
sexual, 35 ff-. 481. 525 n., S 45 . 555
social, 479
types of, 125
Willis’s views on, 204
Selection-pressure, 117, 230, 324, 447,
SH

intensity of, 475

Selective advantage, and intensity of
selection, 56

Selective depression, 447
Semispecies, 403, 407
Separation, altitudinal, 270
geograpiiical, 270
Sex, consequential effects of, 545
Sex-chromosomes, peculiarities of, 138
Sex-determination, 64, 97, 148 ff.
Sex-differentiation, in frogs, 235
Sexuality, evolution of, 83
Sicherung^ doppelte, 108, 417
Sightlessness, and preadaptation, 453
Size, and speciation, 280
Society Islands, land-snails of, 232
Song, biological function, 289, 298, 534
clmein, 215
geneticai basis of, 305
geographical variation in, 183, 309
learning of, 306
Spain, camels of, 236
Specialization, 42, 84, 488, 567
in mammals, 490
Speciation, 43
and evolution, 387 C
and habitat-preference, 254
and intensity of selection, 38 3
and mobility, 155, 239
as a biological luxury, 389
convergent, 385

different modes of, 170 ff, 382ff.
divergent, 385
geographical, 174 ff.

Goldsdunidt’s views on, 197
modes of, and systematic method,

390 ff.

reticulate, 171, 385
successional, 172, 385
types of, 155

Species, as biological units,; 1 51, 167,

169

biological, 274

biological reality of, 151 ff., 167, 169
criteria of, 159 ff.
cryptic, 130, 299, 300
deffnidons of, 157 ff., 167
ecogeographical, 270
immutability of, 390
kinds of, 154 ff, 382
monotypic, 407
morphological, 409
number descried annually, 169
number of, 168, 389,
numerical abundance of, 479
Origin of, 153
origins of, 387
physiological, 315
polymorphic, 407
polytypic, 407
properties of, 165
rare and abundant, 32, 197
relativity of, 244
subsexual, 351, 383
type of structure favouring evolu-
tionary change, 60
utility of term, 170
Species-formation, see Speciation
convergent, 339 ff.
reticulate, 35 C
types of, 382 ff.

Species-hybridization, 115, 146 ff.
Species-pairs, 273, 280, 309, 334> 385
infish, 181
overlapping, 284 ff.

Specific modifiers, origin of, 75
Spermatheca, effects of white gene, 80
Stenohaiine forms, 444
Stenoplastic forms, 444, 519
Stenothermic forms, 444
Stocks, divergence, 68
Straggle for existence, 14
Straggle for survival, 15
Subspeciation, adaptive, 182, 192
and migration, 196
and mobility, 239
nonradaptiye, 1 93
rate of, 194
Subspecies, 408
biological, 298, 312
chains of, 243
dependent, 230, 229, 260
ecological, 230, 277
Goid^hmidt’s views on, 197

622 ■, , byolution: . thb modern synthesis

Siils^cies {cmtinmil
indepeadentj 210, 260
local, 33* 3 i 9 E

: poiyphyletic, 209, 248, 291
regional. 198
Subsubspecies, 202
Succession, ecological, 276
Superspecies (Supraspecies), 179 4^7 ;

Sunrivai. Stn^gle for, see Struggle
Symbiosis, 312, 429
Synaposemati^m, 321
Syncytia, 170

Systems, genetic, 126, 359 ■

genetic, partial, 67, 139, 33^, 3<53
Systematics, see Taxonomy
comparative, 226, 241
mctbodoiogy of, 409
minor, 43

Tameness, genetic, in birds, 310
on islands, 243

Tanganyika, ralBation in, 324, 492
fauna of, 181

Taxonomic criteria, 390 C
Taxonomic groups, special types of,

31^

Taxonomic terminology, misuse of,

409

Taxonomic units, 407
Taxonomy, 30, 226, 241
approadies to, 390 C
geographical, 264
history of modern, 390 ff.
practical aims of, I $6
terminology of, 404 ff.
thenew, 411
theoretical aims of, 157
Telepathy, 574

Temperature^resistance, 191, 235, 314,
43 < 5,451

Terminology, subsidiary, 157, 163,
216, 405
misuse 01^409
Territory, of birds, 289
Tetraploidy, 87

Threat, characters subserving, 35
Thyroid, and metamorphosis, 553
subspecific differences in, 188
Time, and spcdarion, 173, 194, 200, 324

Topoclines, 223 .

Tradition, in man, 573
Transference, of sex-characters, 525 n.,
■ 545 , 555

Translocation, 90, 330, 362 ^

Trends, apparent orthogenesis of, 497 ff.
adaptive, selective determination of,
494fi'. '

evolutionary, 42, 486 It
evolutionary, consequential, 543 C
non-adaptive, 504 C
paralel, J47
unilinear, 172
Trimorphism, 97 » 3:03, 122
Trisomy, 89

Types, prime, 90* 198, 3^9

Ultracytoiogy, 357
Use, inherited e&cts of, 17 ^

Useless structures, degeneration of, 476

Variability, 54, $6
and fluctuations, 1 12
determinate, 548
inherent, 238

Variance, and type of inheritance, ^5
in large and small species, 57
Variation[s], 14, 16, 17fi*.
continuous and discontinuous, 23
in allopolyploids, 145
mendeiising, 117
oceanic, 325

parallel, 99, loi, 211, 215, 395,

$10

restriction of, 516 fil
types of, 46

Vertebrae, dines in, 215, 223
Vestigial gene, change in expression of,
71 n.

effects of, 1 19
Vestigial organs, 455, 530
ViabEity, 190
Vitalism, 465

Wild type, stability of its characters, 73
Wind, and winglessness, 120
Wing-form, dines in, 215
Wing-length, cHnes in, 213

Mantis kvehu, 513
Abramis brama, 273
Abraxas ^rossulariata, 83
Acanthiza, 214, 433, 516
Acanthopneuste piridana^ 444
Accipiter gentilis, 218
AccipUer nisus, 282
Accipiter novaehollandiae^ 184
dimorphism, 106
Accipiter ventralis, 184
Achatinellidae, 234
Achillea, 273
Achillea millefolium, 441
Achroia grisella, 302
Acraea Johnstoni, 217
Acrididae, 431

Acrocephalus schoenobaenus, 309, 534
Adalia frigida, 548
Adapinae, 515
Afgilops, 345

Aegithalos caudatus, 247, 280
Aepyornis maximus, 506
Acpyomithidae, 506
Aesculus carnca, 341
Aesculus hippocastanum, 341
Aesadus papia, 341
Aesculus plantierensis, 341
Aethusa cynapium, 278
Ailuropoda melanoleuca, 427
Ailurus fulgens, 427
Ajuga^zjj

Ajuga chamaepitys, 180, 267, 398
Ajuga chia, 180, 267, 398
Alauda arvensis, 306, 426
Akelaphus, 2SS

AlchemiUa aipina, 266
AlchemiUa pulgaris, 266
Akuosmia, 3 53 » 355
Algae, 132, 135
Aloe ciUariSy 347
Alopex,iJ%

Alopex kgopus, 103,161
Amblypods, 491
Ammomanes deserti, 462
Ammonites, 172, 507, 530, 567
Amphibia, 503, 505, 545
Amynodonts, 498, 506
Mas platyrhyncha, 239

Anatidae, 240
Anemone, 273
Anemone alpma, 273
Anemone nwntana, 336
Anemone nemorosa, 477
Anemone sulphurca, 273
Angicr-fish, 1 59 11.

Animals and plants, difference in
genetic systems, 13 s
Annelids, tiumbcr of species, 168
Annulata, 144
Anopheles macuUpemns, 317
Anoptidithys jordani, 454
Atser cocrulescens, 184
Anteaters, 419, 536, 537
allometry in, 537
Antelope, 492
Antennaria, 376
Anthus certdnuSy 180
Anthus pratensis,, iSo, 266, 272, 289,
306

Anthus spinoktta, 279
Aiitlms spinoletta petrosus, 266, 272
Anthus tritnalis, 272, 289, 306
AnthylUs vulneraria, 356, 441
Antirrhinum, 115, 348
Ants, limits of evolutioiiary trend, 494
reproductive specialization, 311
Aonidklla auranti, 471
Aphids, 84
winglcssness in, 74
Apketa nehulosa, 72 n.

Apodemus, 118, 190
Apodemus Jiaincollis, 271, 435
Apodemus sylvaticus, 271, 436
Apple fly, 296

Arachnids, number of species, 168

Archaeopteryx, 31

Ardea cincrea, 443

Argusianus argus, 427

Argya fulvus, 462

Argynnis paphia, 98

Aricia, 253

Artemia, 141

Artocarpaceac, 455

Arum macuhitum, 439

Arum negkctnm, 439

Aster occidcntalis, 441

EVOI,ljrTION:M:HB- modern synthesis

624

Astyanax JasdatuSf 4$4

Austraiia, fauna of, 324, 49011., 491

Avmafatm^ 332

Avma satipaf 332

Axolotl, 527

Babbler, 462
Baboon, 501
Baaeria, 13 1, 301
biological differentiation in, 302
BacuHtes, 507
Baetis, 435
Balafioglossus, 23
Balsam, 107
Baluchitheres, 491, 498
Bandicoots, 423
Barbet, 195, 551
Barley, 63, 93
Basidiomycetcs, 135
Basilarchia, 253
Bats, 423, 490
subspeciation in, 239
Beans, 52, 1 18, 130
Bear, 103
Beaver, 488
Bedstraw, 121, 273
Bee-eater, 31 1
Bee-orchis, 477
Bees, 220

Beetle, ladybird, 548, 549
Beetles, 298, 299, 536, 541
Bell-heather; 278
Berberis, 301
Berlinia, 439

Bird of Paradise, 240, 425, 426, 427
Lesser Superb, 425
Birds, difScult species in, 167
display adaptations in, 425
flightless, 129
genetic tameness in, 310
nest sanitation in, 424
of prey, 424
polymorphism in, 103
subspeciation in, 176
Bismtella laevigata^ 337
Bittersweet, 277
Blackberry, 334

Blackbird, 266, 290, 3od, 307, 309
Blackcap, 306, 307
Blackcock, 36, 289
Bladder-campion, 266, 268
Bladder-worm, 485
Blight, 198
Bluebell, 514

Baarmiaextersma.gs
Boarmk repmdata, 95, 470
Bomhim, 246
Bomhinatoft 246
Bamhus, 214
Bombyx quetcus, 293
Bonellia,iS9n.

Brachiopotfa, 508, 557
Brachystegia, 439
Bracken, 517
Brambles, 3 51
Brassica, 350
Brassica oleracea, 347'

Bream, 273
Brcmm tnemiSf27S
Broom, 277
Brush turkey, 255

Bryophyta, 135
Buarremon, I 99 » 242
Buarremon inomatus, I 99 i
Budgerigar, 307
Buffalo, 192, 214, 218
Bufo fowleri, 253
Bufo woodhousit 253
Bi^ie, 180, 267
Butims contortusy 312
BuHfmch, 247, 254, 281, 327
Bunting, reed, 278
yellow-breasted, 444
Burnet, 293
Burrageara, 345
Bush-baby, 428
Bustard, great, 426
ButeOy 355
Buteo borealis, 252
Buteo buteo, 282
Buteo galapagensis, 310
Buttercup, 199
Butterflies, 74 n,, 96, 262
blue, 285, 290
mimicry in, 122
white, 518

Butterfly, American Clouded Yellow,
98

buck-eye, 244
comma, 445
Small copper, 217
Swallowtail, 1 91, 217, 225, 278
White Admiral, 253
Buzzard, 282, 310

Cabbage, 347, 350
Cabbage-radish hybrid, 1 41, 347
Cactospiza. 326

INDEX— ORGANISMS

Caliosciurus, 193
Calhsciurus sladeni, 219, 227
CamarhynchuSf 326
Camel, 236
Camelim satwa, 115
CamperkUa bifasdata, 299
Campion, 142
Canary, 305
Canidae^ 294, 502, 539
Caninae^ 351
Cants familiarist 539
Cams iupust 540
Capsella, 346
Capsella bursapastcris, 347
Capsella ocddentalis, 347
Carabus granulatus, 176
CarabusmoniUSf 180
Carabus nemoralis, 206, 235, 314
Carduelis cannabina, 266, 306
CardueUs carduelis^ 187 n., 194, 281,
290

Cmduetis fiamtneay 212, 290
Catiudis fiavirostrisy 266
Caribou, 273
Carnivores, 490
Caryophyllaceae, 205
Cassowary, 476
Casuarius, 476
Cat, 103
Siamese, 64, 546
Catastomus, 540
Catfish, Nile, 415
Cattle, 541
Caucaiis arpensis, 27B
Cave-bears, 506
Capta, 501

Cedar of Lebanon, 438
Ckdrus UbanU 43%

Celandine, lesser, no
Centaurea, 147, 258
Centaurea nemoralis, 441
Cepaea, 202, 520, 532,

Cepaea hortensis, 99, 161, 291, 516
Cepaea nemoralis, 99, 161, 291, 516
Cerambycidae, 298
Cercopithecus mona, 247
Cercopithecus polykomas, 247
Cerihia hrackydactyla, 245, 306
Certhia familiaris, 245
Certhidea, 326
Cervidae, 541

Cervus elaphus, 121, 225, 537
Cetacea, 240

Chaffinch, 183, 306, 307, 309

Chalcotheres, 491
Char, 177, 180, 231
Chat, 191
Chelonia, 505
Chickadee, 180, 270
Chickweed, 5x7
ChifFchaffi, 278, 289, 306, 307
Chimpanzee, 53
Chloeon dipiemm, 435
Chloris chloris, 306
Chorthippus, $16
Chrysanthemum, 348
Chrysohphus amherstiae, 66
Chrysomphalus aurmtii, 299
Chub, 348

Ckadulina mobile, 312
Cichlids, 324, 493
Cichorieae, 377
Ciliates, 84
Cindus, 434
Cinnyris mamcnsls, 272
Cinnyris zonarius, 272
Cisco, 178
Cistkola, 176, 289
Clausilia, 448
Clausilia hidentata, 246
Clausilia dubia, 246
Cleithrionomys, xi8
Cleithrionomys glareolus, 1 1 8
Clypeolajonthlaspi, 197
Cob-antelope, 234
Cocdnellidae, 220, 550
Coccothrmstes coccothraustes, 281
Coccus pseudomagnoUarum, 471
Cockchafer, 312

Coelenterates, number of species, 168
Coereha, 184, 203
melanism, 104
Coerehajiavcola, 94 n.

Colaptes, 161, 250
Colaptes auratus, 288
Coleoptera, sex-determination in, 149
CoUas, 516

Colias pkilodice, 98, 117
Columba oenas, 310
Compositac, 205
Conepa!tus,$4%

Caradas, 252
Coradas garrulus, 444
Corals, 536
Coregomis, 178
Coxixidae, 468
Cormorant, 310
ffightiess, 243

636

KVOiUTIOISi:'' THE MODERN' SYMTHiiSIS

Corophum poluiaton 53.6
Corvidae, 21 1 ■

Corpus^ 248 - 9 , 262 .

Corpus brachyrhptekoSf 280
Corpus £orax partus, 262
Cortfus ossifragus, 280
Cotton, 54, 70, 77» ii5» ^^ 6 , 228, 346,
45% 533
Cowries, 177
.Cowslip, 275
Cmmbus, 292
Craiaegm, 296 , 351 '

■'Greodonts, 490 .

yCrepis, 128 , 13 % 14 % 350, 353» 35^. 3^1
spcciatioii in, 372 ff. . -
Crketm erketus, 103 ' ■

.Crickets,. , 298 , 309
Cromia cromm, i% 3 '

Crossbill, 281

Crows, 248 , 255 , 280 , 403

Crucifers, 346 ...

Cnistacea, 84 , 85 , 559
number of .species, 168
■. preadaptation ill, 455
Cryptostilis, 467 . '

.Cuckoo, 102 , 309 , 3 II
.■ ■ egg-mimicry in, 451
CucIcGO-pint, 439
Cumius -emorus, 102 , 309
Cukx pipiens, 319
Curlew, 281 .

Cuttlefish, 416
Cyclopes, sz 7 ^

Cydia pomonelia, 472
Cyllene pictus, 298
Gynipidae, 285 , 348 ^

Cynips, 299
Cypems dentatus, 388 11 .

Cypraeidac, 177

Cyprinodonts, sex-determination in,

,.'. 14 ^^ '

Cytisus scoparius, 277

Daifodil, 438
Dahlia, 144
Dahlia merckii, 349
Dandelion, 120 , 166 , 477
Daphnia longispina, pre-adaptation in,
52,60
Daiura,()o

Datura stramonium, 89 , 198 , 329
Deer, 484
red, 121 , 225 . 537

Dccr-itiouse, 54 ,. 176 , 182 , 291 , 294

DeUchott urbka, 279

Deronectes, 247

Dianthoecia, 520

Dingo, 82

Dinosaurs, 446 , 506

Dipnoi, 544

Dipodomys, 176 , 423

Dipper, 434 ' ■ ■

Diprion poly tomum, 314
Diptcra, 142
Dog, 115 , 307 . 506 , 539
allometry in, 539
dominant characters in, 82
St. Bernard, 71
Donax, 520

Drepanididae, 183 , 324 , 457
Dromaeus, 476

Drosophila, 47 , 48 n., 50 , 51 , 52 , 53.
55 , 62 , 63 , 64 , 67 , 69 . 74 . 75 /^ 0 ,
85 , 89 , 90 , 91 , 92 , 115 , 117 , OH,
119 , 120 , I2I, 133 , . 138 , T 42 , 148 ,
152 , 189 , 206 , 288 , 291 , 292 , 330 ,
33t. 35^. 357 ff., 395. 39d, 4 m*
45% 453. 459. 4^^% S^o, 51 % 5^2.

514.533.545^

Drosophila, speciation in, 357 C
wild variance in, 75

Drosophila amermna, 5 $, ^67
Dfosopkilajmehris, 191 , 235 , 314 , 436
Drosophiia hydei, 53 , 61 , 370
Drosophila tnvlamgaster, 68 , 70 , 71 n.
72 , 75 11 ., 97 , 1 13 . 162 , 199 , 303 ,
314 . 333. 3«3
Drosophih miranda, 93 ,■ ■

Drosophila obscura, 70
Drosophila pscudoobscura, 6q, 93 , if? 2 ,
194 , 288 , 323 . 333. 359. 364 , 369 .

.405 .

Drosophila simulans, 72 , 162 , 333 , 383
Drosophiia virilis, ^ 6 , 358 , 367
Dryasoctopetak,$ty
Dryobates major, 2 ^ 0 , 44 S
Dryobates minor, zBo
Dryobates puhescens, 280
Dryobates vtllosiis, 280
Duhyaea, 377 , 378
Duck, 146 n., 240 , 292 , 516
steamer, 285
tufted, 239

Echiiiodcrms, 172 , 522
number of species, 168
Echinogammarus, 328

INBIX— OSGAMISMS

Echium pulgm^e, 107
Edelweiss, 274
Edentates, 49O:

Egret, lesser, 425 ■

Egretta thule^ 425
EiiohsmmSr 505
Elasmobraiidis, 544
Elephaiat[s], 172, 488, 490, 499, 501
pigmy, 121
Elk, Irish, 541
Elms, 227

Emheriza aureola, 444
Emberiza dtrimUa, 306, 309
Emheriza schoenkhlus, 278
Emmers, 345 :

Emu, 476
Biagrostis, 337
Encyrtus infelix, 428
Ephemera vulgata, 43 5
l^hestia, 121
kuhniella, 68, 302, 450
Epikchna chrysometina, 549
Epimachus paradisea, 42s
Equisetaceae, 455
Equus burchelli, 217
Erehia, 176, 183
Eremophilajiava, 445
Erica dnerea, 278
Erica tetraUx, 278
Brithacus ruhecula, 36, 306, 309
Ermine moth, 297
Erophita verm, 336
Eumenes maxillosus, 320
Euphrasia, 282
Eurypterids, 560
Exocoeddae, 542
Eyebright, 282

Falco coluwhariiis, 280, 282
Falco peregrims, 280, 282
Fake rusiicolus, 221
Falco subhuteo, 282
Falco timunculus, 282, 310
Falcon, 280
Felidae, 422
Felis onca, 281
Felis pardalis, 281
Felsinotherium, 505
Fern, 33, 55, 198
Fiddler-crab, 541
Field-mouse, 435
long-taile4 271
Filipendula hexapetala, 44'’

Firccrest, 290
Fish, 146 n.

differentiation and habit, 241
subspeciation, 177
Flamingo, 36

Flatfishes, asymmetry of, 456
FEcker, 161, 230, 255, 288, 354,
Fiueliin, 438
Ply, 148

Flycatcher, 222, 310
collared, 284
pied, 284
Flying-fish, 542
FooFs parsley, 278
Forairmnfera, 408
Fowl, 53, 59

Barred Plymouth Rock, 450
dominance in, 72
dominant characters in, 81
frizzled, 63, 76, 118, 190, 315
Rhode Islajid Red, 450
White Leghorn, 450
Fox, 97, 103, III, 217
Araic, 103, 1 61
black, 185
red, 103, 185
relative ear-size, 213
silver, 185

Fratercula mtka, 212
FrittgiUa coekbs, 183, 306, 308
Ftingilla coekhs canariensis, 255
Fringilla teydea, 255
Pritdiaria, 339 n.

Fritiliary, greasy, fluctuations in,
heath, 195
Frog, 235, 4^9, 435
relative leg-size, 312
Fruit-dove, 285
Frutidcola lantzi, 315
FutmarusglaciaUs,2i7,44.s
' Fundulus, 544
Fungi, 307

Galago, 42$

Gakopithecus, 423
Gakopsispuimcens, 341
Gakopsis spedosa, 341
Gakopsis tetrdiit, 341, 384
Galerida, 192, 215, 284
Gakrida aristata, 215, 284, 445
Galerida tbeklae, 215, 284
Odium, 121, 375
Galium saxtik, 273

628 evolution: THE:,MODEItN SYNTHESIS

Galium sflpestref2*js y
Gallimgo^ 240
Gall-wasp, 348
Gammarids, 493
Gammams, 70, 75, 528, 533
wild variance in, 75'

Gammams dieweuxi, 51^

Gammams duebmi, 279
Gammams tigrinus^

Gamimms zaddachk ^1$

GarruluSf 176 , 179
Gastropods, 543
Gennaeus^ 21B^ 224
Gentian, 177, 273, 514
Gentiwta cksti, 273
Gmtiana exdsa, 273
Gentimta lukari77
Geospiza^ 326, 3$6
Geospizidae, 242, 290, 325, 356
^Geranium, 2 %o
Gerrhmotus, 214
Gipsy-moth, 191,436
Giraffe, 421
Glasswort, 276
Gkekoma, 108
Gbssina morsitans, 440
Glossina palpaUs^ 440
Gbssina st4bmorsHans, 439 n.

Gbssina tachinoides^ 440
Glypta haesitator^ 425
Gnat, 319
races, 317
Gobies, 325
Goidcrest, 194, 290
Goldfinch, 187 n., 194, 281, 290, 307
Goose, Hawaiian, 421
snow, 184
Goshawk, 218

Gassypium, 115, 116, 228, 533
Gossypium arboreum, 78, 346
Gossypium barbadmse^ 77, 346
Gossypium hirsuium, 77
Gossypium tkurberi, 346
Crackle, 251, 293
Grape, 314
Grass, cocksfoot, 276
rice, 341

Grasses, 108, 337

Grasshopper, 99, 202, 253, 321, 431,
473 , 516
i%htiess,202
Greenfinch, 306
Groimd-finchcs, 242, 290, 326
Ground-sloths, 490

Grouse, 427

Grouse, red, 176, 196, 266, 271, 378,
293,521
sharp-tailed, 219
willow, 196, 271, 293
Grouse-locusts, 99, 473
Gryphaea, 489, 506, 508, 509, 514,
5150., 537

Guillemot, 105, 161, 217
Guinea-pig, 501, 550
Gull, coloration ill, 518
herring, 244
lesser black-backed, 244
Gymnospemas, absence of polyploidy
in, 145

Gyrfalcon, 221
Gyrimst 247

Haartebeest antelopes, hybridization
in, 253

Habrobracon, 149
Hamites^ 508
Hamster, 53, 103
Hare, 246, 266, 271
common, 246, 266, 271
mountain, 246, 266, 271
Hamonia axyridiSt 214
Hawfinch, 281
Hawk, 103
red-tailed, 252, 355
Hawk-moth, spurge, 133, 312
Hawkweed, 277, 334, 353, 35^
Hawthorn, 296, 351
Heath hen, 201
Hek, 353, 355, 393
Hedgehog, 490
Helianthemum, 108
Helbsciurus gambianusj 192
Helix aspersa^ 520
Hellebore, 447
I Helkborus foetidus, 447
HemicenteteSf 2S1, 2^6
Hemkentetes semispinosust 544
Hcmiptcra, sex-determination in, 149,
370

Hemipus pkatus^ 41s
Hemizonia, 276
Hemp-nettle, 341
Henkopernis, 223
Heodes, 217, 225
Heodes phbeas, 217
Hepiaius^ 290
Heron, 103, 443
Louisiana, 425
Herring, 177

INDEX— ORGANISMS

629

Heterodera radkota, 300
Heteroptera, 339, n. 370
HeterorhYttchuSy 325
Hieradunty 334, 377
Hieracium umhellatumy 277
Mippanofty 502
Hiradmea, 335
Hirundo rustkay 279, 311
Hobby, 282

Homo neandcrthalensiSy 354
Honey-bees, 220
Honey-buzzard, 223
Hombili, 428, 536
Hornets, 322

Horsefs], 32, 172, 396, 488, 492, 502,
5 i 4 » 536

in Nova Scotia, 121
rate of evolution in, 61 11.
Horse-chestnut, 341
Horse-tails, 455
House-wrens, 181
Hoverdy, 323
Humming-bird, 232, 426
Hyacinth, 438
Hydranassa tricolor ^ 425
HydrocyoHy 324

Hyena, 183, 545
Hymenocephalus striatissimusy 177
Hymenoptera, sex-determination in,

149

social, 74, 96

social, and Lamarckism, 461
Hyperimmy 3$o
HypoUmnas dubiusy jloi
Hyponomeutay 298
Hyponomeuta padelky 297

IberPiUea sonorae, 442
Ichneumon-fly, 485
Ichthyosauria, 505
Icterus gaibuky 305
Ictonyxy S4S

Impatkns nolkme^iangerey 107
Insectivores, subspedation m, 239
Insects, flightlcssness in, 453
number of species in, 168
social, selection in, 43, 480, 4^2
types of speciation in, 322

lOy 431

IfiSy 224

.:Jagmr,:" 28 i •

Jay. i76y i79» 180

Jimsoii weed, 89, 198
Jird, 283

Juglans nigray 437
Jtmcoy 248
JuncuSy 108
Junonia tavmiay 244

Kangaroo, 492
tree, 448

Kangaroo rat, 176
Kestrel, 282, 310
Kingfisher, 424
Knapweed, 258, 441
Koala, 283
KobuSy 234

Labyrinthodonts, 504
Lacerta muralisy 283
Lacerta skulay 200
LachnantheSy 189
Lactucdy 377

Lady-beetle [s], 210, 214, 220

Lady’s mantle, 266

Lagopus lagopusy 176, 196, 293

Lagopus mutuSy iii, 196, 266

Lagopus scotkusy 176, 196, 266, 293

Ldage aureOy 199

Lamiumy 108

Lamprey, 282, 315

LamprodituSy 328

Lamp-shell, 567

Land-snails, 223, 232, 242, 543

Land-tortoises, 242

Lmius, 22$

Lanius collurio, 252
Lanius ludopidamSy 236, 238
Lapwing, 451
Lark, Clot-bey’s, 422
crested, 192, 215, 284, 445
desert, 462
shore, 445
Lams argentatuSy 244
Lams fuscuSy 244
Lasius niger, $22
LateSy 324
Leaf-hopper, 3 12
Lebistcsy^

Lemming, {Lemmus), iii, 114
Lemur, 422
flying, 423
Lemuroidea, 515

Lepidoptcra, and green pigment, 517
Lepto^ermtmty 355

630 EVOiUTlOKr.-THE MODERN SYNTHESIS

Lepus amerkmus^ in
Lepus europacus^ 246, 266
Lepus timidusy 246, 266
LeuckhthfSy 178
Lemkkthys artediy 17H 11.

Leucodonta bkohnay 106
Lke, 305 ■ ■

LimenitiSy 253
Linmaea columelUy 236
Limmimn pyramMey 276
Lmmtimn mnftoray 276
Lmontum tmlgarey 276
Limpet, 319
Linaria spumy
Liuqula, 205, 557, 567
Linnet, 266, 306
Lktorim okusutcty 222
Lizard, 200, 214, 232, 283
agamid, 503
Lobelia, 232
Lobiophasis hulwcriy 425
Lohipluma malabarkay
LocustcUa naeviay 306
429

Lomechusciy 467
Longicom beetles, 523
Loosestrife, 53
Ijyphoaros mclamJcucoSy 428
Lophophorus impeyanusy 66
Lophoritia superbuy 425
Loricariidac, 503
Louse, 305
Loxia curvirostmy 2S1
Lucanidac, 537, 541
Lucerne, 342
Lusdnta luscimUy 246
Lusdnia megarhyncfiay 246
Lybiusy 195, 551
Lybius torquatuSy 19s

Lycaena corydetiy 2?>s
Lycmta thetisy 2$$

Lycaenidacy 280

Lymaniriay 115, 220, 235, 314, 323,

442,533

Lymantna disptiry 4 j6
dines in, 216
LynxyXix
Lythruniy 53

Malaria, 317 ;■ ,

Mallard, 239

Mammals, poiymorpbism in, 103
Scottish, 183
subspcciatioii, lyd
toilet-adaptations, 422
Man, 50, 59, 507
as dominant type, 561 C
blood-group dines in, 219
correlated characters in, 534
evolution of, 526
haemophilia, 55
popiilation-structtirc, 6i
reticulate evolution of, 354
selection in, 129
taxonomy of, 403
Manatees, 490
Mantis, 102, 485
Marsupials, 324, 423, 49011., 491
Martin, house, 279
sand, 279
Mastomysy 258
Matthwla incanay 189
Maydy, 435
Meadowlark, 251
Meal-motli, 68, 303, 450
Medkagi} Jalcatay 342
Medkago satwUy 342
Medkago sylmstrisy 342
MegapodiuSy 255
Mcgascopsy 106

Metampsora ribesikpurpureay 301

Melandriunty 142

Melitaea atkalkiy 195

Melitaea mrinWy 112

Mellivora, 548

Mehspiz a melodiUy 21^ y 272

Mephitisy $4%

MeretriXy 520
MerioneSy 283
Merlin, 280, 282

Memps apiastefy 311
Mertensky 441
Mesia argmtauriSy 215
Mesmaurusy 505

Metazoa, number of species, 168
Mctridium senile y 100
MicrasteVy 32, 172, 174, 396, 514
MkrotuSy 105, 212
Mkrotus arvalisy 105, 203
Milkwort, 438
Millet, 339 n,

MimidaCy 242
Minivet, 289

MacheteSy 427
Machetes pugmXy 102
MahonWy 301
Maize, 328. 331, 339 n.

INDEX-ORGANISMS

Mistletoe, 299, 308
Mites, mange, 300
sex-determination in, 149
Mocking-birds, 242
Mole, 421, 490
marsupial, 492
MoUiemsia formosa, 316
Mollienisia Jatipinna, 3 16
MoUienista sphenops^ 316
Molluscoidea, number of species, 168
Molluscs, number of species, 168
Monkey, Colobus, 247
Mona, 247

Monkeys, hybridization in, 253
Mosquitoes, races in, 317
Moiadlk, 176, 179
Moth, 93, 106, 120
codling, 472
currant, 83
gipsy, 191, 43Z
Moiiflon, 542
Mouse, 76, n 8
hairless, 71
house, i87n.
house, Faeroe, 195
pleiotropism, 62
Mouse-deer, 183
Mulkrornis, 506
Munia flaviprymna, 433
Mas faeroensiSf 195
Mus musculus^ 118, 187 n.

Musdeapa albkollis, 284
Musdeapa hypckuca, 284
Mussels, 431
Mustek^ 255
Mustek ermineusy 281
Mustek nivaUsy 2^1
MustelidaCy 281

Myriapods, number of species, 168
AfynWms, 310
Myrmecophagay 537
Myrmecophagidae, 537
Myrmka ruhruy 322
Myxobacteria, 328
Myxosporidia, 300

Narcissi, 336
Nasturtium, 71
Nasutitermes guayanae, 312
NautihiSy 205
Nemerites canescensy 302
Memobiusfasdatus, 298
Nephrodium spimhsumy 33, 198
NephrolepiSy 55

631

j Nesochen sandmeensiSy 421
j fJeHion creccay 238
I NeuroteruSy 285
I Nicotianay 3 son,

I Nkotiana diglutay 344
I Nkotiana diglutosOy 344
Nkotiana glaucOy 343
Nkotiana glutimsay 344
Nkotiana kngsdorffiiy 343
Nkotiana multivalensy 344
Nkotiana panicuktdy $44.

Nkotiana rusticay 344
Nkotiana suaveolensy 344
Nkotiana syivestrisy $44
Nkotiana tahacumy 145, 344
Nkotiana tomeniosay S 44
Nkotiana trigondphylky 345
Nightingale, 246, 254, 288, 305, 306
northern, 246
Nighgar, 413. 416
Notharctinae, 515
Nucifragay iii
Nudfraga caryotaetesy 114
Numenius arqmta, 281
Numnius phaeopus, 281
Nutcracker, iii, 114
Nuthatch, 219
Nyctea nyctea, 114
Nyctibius griseuSy 416
Nyroca fuUguky 239

Oak, 274
Oak-eggar, 293
Ocelot, 281
Oecanthus nipalis, 298
Oenanthe tugensy 191
Oeneis chryms, 463
OenotkeraySJ^ I37» I 39 » 150, 3 ^ 9 , 3 ^^
Oenothera hookerU 91
Oenothera lammckimia, 189
Oligochacta, 335
Ophrys apiferay 4JJ
Oporabia autumnatay 195
Oporiniamtumnatay 120
Oporomis Philadelphia y 180
Oporomis tolme^ J%o
Opossum, Tasmanian brush, 53, 104,
203

Opuniky 300
Orchid,^ 449, 467
Oreotrochilus chimboraziy 232
Oriole, Baltimore, 305
Orycies nasicomisy 176
Osprey, 282

632 evolution: the moi>hi?n synthesis

Ostmcodcrnis, 560

Ostrea, 4H% 51" 5 il, 537
(Istrich, 476
Otis tardti^ 426
'Oiizel, ring, 266, 290
Owl, i03, l'06
Snowy, 114
Oxalk, 108
Oxlip, 274

Pacliyaphis, 505
Pncltypiema^ 505

spcciatioii in, 381
Faint-root, 189
Pabcoisisddae, 542
Palconiscoids, 158
Falmaceac, 455
Palms, 455

Paludesirim jcnkinsi^ 313
Panda, coniinoii, 427
giant, 427

Pandion luiliaeius, 282
Pansy, field, 517

Papilh dardanus, 123, 159, 191, 217
Papilui hector, $1$

Papilio mackaon, 176, 225, 278
Papilio memtion, 97
Papilio polytes, 97, loi, 513
Papilionidac, 262

Paradise, Birds of, 240, 425, 426, 427
Paradiscidac, 240, 425 ff.

Paramecium, H4
Paramys, 487, 488
Paratetrmychus, 300
Paratettix texams,

Pariula, 202, 232
Partnla clara, 233
Piirtula mirahilis, 233
Partula otaheitofm, 2^^

Partula suturalis, 2$^

Pants, 176

Parus atcr, 218, 269, 280, 290, 309
Parus atricapilfus, 180, 270, 280
Partis caroiinensis, 180
Parus cristaius, 26ij
Parus major, ?eS5i 309
Pants nitier, 272 ' •

Parus palustris, 270

Passer domcsticus, 94 11., 256, 279, 448,

519

Passer hispmiiolensis, 2$6
Passtr montamts, 279
Passerclla, 220, 225

Passerella iliaca, 1H2, I9(), 238
Passerelia fmoinii, 23^

Passcrelkmehdta, 23S
Paielin, 319
Pai^o cristatus, 427
Peacock, 36, 426, 427
Pedimhts, 305

Pedioecetes plmshnellus, in)

Pciigiiiiis, coloration in, 518
Peony, 381
Peramclidac, 423
Perch, 158
Peregrine, 280, 282
Pericrocopus, 289
Periophtkalmus, 553
Perisoreus canadensis, 180
Perhoteus ohscums,

Periwinkle, 222
Perognailms, 462

Peromyscus, 54, 9611., 11.5, 176,182,
188, 189, 190, 193, 206, 213, 216,
224, 229, 235, 291, 294, 432,

434, 462, 479

Peromyscus kucopus novebimeensis, 225
Peromyscus mankulatus, 188, 189, 217
Peromyscus poUonotus, 1 ) 36 , 208, 210,
218

Petrel, fulmar, 106, 217, 445
Petromyzoti, 282
Phascolarctus cinercus, 2S3
Phase0lus,ii$

Phaseolm muhijioms, 343
Phaseolus vulgaris, 343
Phasianidac, see Pheasants
Phasianus cokhkus, 180
Phasianus pcrstcohr,

Phcasant[s], 14611., 252, 294, 425,426,
427. 521
Argus, 427
Bulwer’s, 425
Iinpcyan, 66
Japanese, 180
Lady Amherst, 66
silver, 224

Phoenkurus ochrurus, 266, 445
Phoenicurus phoenkurus, 266
Phylloscopus, 176

Phylloscopus coUyhha, 2'7^, 289, 306
Phylloscopus plumbeitarsus, 245
Phylloscopus sihilatrix, 2H9
Phylloscopus trochilus, 278, 2H9, 309, 424
Phylloscopus uiruhmus, 245
Phyiodccta, 548
Phytophthora infest ans, 198

INBEX — OEGAMISMS

633

Piddae, 2II '

Pims^ 223

Pirns cams smguinkeps, 215
Pietidae, 262
P%, 189, 481, 542
Pigeons, 450, 496
Pine, Scots, 438 ■

Pirns, 224
Pirns sylvestris, 438
Pipit, 180

meadow, 266, 279, 289, 306
rock, 266, 272, 278
tree, 289, 306
Pituophis, 540
Pii^i^ysmus, 299, 523
Planarkm, selection in, 121
PImtago Imceotata, 107
Plmtago major, 441

Plantago maritima, 177, 207, 223, 275,

277

Plantain, 107, 441
Plants, ecotopic divergence in, 273
subspeciation in, 177
Plants and animals, difference m genetic

systems, 135
Plasmodium vivax, 319
Phtypoecihs, 66
Platypoecilus maculatus, 100
Platysamia, 219
bybridization in, 253
Platysomidae, 542
Platyspiza, 326
Plesiosauria, 505
Plusia moneta, 445
Poa alpina, 275
Polecat, Cape, 548
Polygala calcarea, 438 ^

Polygonia comma^album, 445
Polypterus, 544
Potnaea, 222
Pomoideae, 350
Pontaniay 304
Pontania salicis, 302
Porpoises, 489
Potato, 336

Potentilla, 337, 347, 441
Potentilla argmtea, 347
Potentilla cdlirut, 347
Potentilla crantzii, 347
Potentilla tahernaemontani, 347
Potinara, 345
Prairie clddien, 201, 426
Prenanthes, 377
Prickly pear, 300

Primates, 490, 515, 518, 526
evolution of, 396
Primrose, 53, 222, 274, 313
evening, 91 > 2:89, 329
Primula, 53, 108
Primula elatior, 274
Primula Jlorihmda, 87, 142, 340
Primula kewensis, 87, 141, 142, 144,
340, 347

Primula sinensis, 64, 107, 189
Primula veris, 275
Primula vertkillata, ^7, 142, 340
Primula pulgaris, 222, 274, 313
Probosddea, 515
Protozoa, 84, 132, 135
Prunus spinosa, ^$6
Psettodes, 497
Psylla mali, 296

Ptarmigan, iii, 196, 266, 271, 278

Pteridium aquilinum, 517

Pteridophyta, 135

Pterosaurs, 446

Puccinia, 301

Puccinia graminis, 307

Puffin, 212

Pu£mus griseus, 286

Puffinus tenuirostris, 286

Pulsatilla, 223

Pygaera, 141

Pygaera anachoreta, 76

Pygaera curtula, 76

Pyrrhula pyrrhxda, 247, ^81

Pyrrhula pyrrhula murina, 327

Quagga, 218
Quercus rohur, 274
Quercus sessilijiora, 274
Quiscalus, 251

Rabbit, 195, 51 532
Himalayan, 64, 54^
snowshoe, in
Radish, 347

Radish-cabbage hybrid, 14 1, 347
Rail, clapper, 273
king, 273
Rattus elegans, 273
Ratlus longirostris, 273
Ramondia, 197
Rana esculenta, 285
Rangifer tarandus, 121, 273
Ranunculus, 339
Ranunculus abortivus, 199
Ranunculus allegheniensis, 199

634 ' " ■"ev'OJLUTIOm:; the

Rmuncutus ficma^im . "

Rmunculus parviflims, 446
Raphmohrasskat 141, 347
Raphmus sativus, 347
Rat, 54, 1 18, 202, 256
black, 256
brown, 256

effects of training m, 459 .
boodcdi, 65, 70 '
pleiotropism, 62, 63
Rate!, 548

Rattlesnake, 308, 537
Rattus (Mastmfs) mucha^ 257
Rattus norv€gm% 256
RmUus rattus^ 256 ^

Raven, 262
Redpol, 212, 290
Redsbank, 402 .

Redstart, Ijiack, 266, 445
comtnon, 266 , ' '

R£gutusignicaptliuSy2^

Reguius reguhis^ 194, 290
Reindeer, 12 1 ' ^ ;

Reptiles, Mesozoic, adaptive .radiation

of. 49J

Rhabdocoela, 334 ■

Rhacopkorus schlegelii, 282
Rkageletis pomonella, 296
RMamphocorys doi-bey^ 422
476

RMnoceros, 485, 498, 535 n.

Rhiptdura hrachyrhyncha, 103
Rliododendron[s], 273, 392
Rho€0, 140, 330
Rhyacia aipkok, 19$

Rhynchospora mpiteUata, 388 n.

Rtbes, 145, 348
Rice-grass, 451
Ride-bird, 425
Riparia riparia, 279
Robin, 36, 242, 306, 309, 310
American, 290, 310
Rodents, 490, $48
coat-colour, 51 1
Roedeer, 442
Roller, 444
hybridization in, 252
Rosa, 147, 35!

hybridizadot-in, 352
Rosa cantna, 383
Rose, 350, 351, 519
Rotifers, 84

sex-determination in, 149

MODERN SYNTHESIS,

Ruhus, 14% 334 * 35 i
hybrithzation in, 352
Rudd, 469

Ruff, 36, 102, 289, 427
Rumex acetosa, 446
Rumex lapathifoUumt 137
Rushes, 388 n.

Rusts, 301, 308

Sacculina, 485, 558
Sagartia,

Smssetiaokaey4’ji
Salamander, 315, 458
Salicaceae, 455
SaUmnia disartmkta, 2’76
Salkomia doUchostachya, 276
Salkomia gradliima, 276
Salkomia Tierbacea, 276
Salkomia ramosissima, 276

302, 345 * 351 . 353
Salmon, 315
Saitator, 199
Sakeiims, 177
Sandpiper, 292, 31 1
Sarcoptes scaber, 3 00
Satyms anthe, $1^

Sawfly, 302

Saxkok rubetra, igo, 307
Saxkok torquata, 290
Scale, black, 471
citricola, 471
red, 299, 471

Semiinius crythropkthalmus, 469
148

SdUa mtummUsi4;^g^

Sdlk pema, 4$^

Sdurus PutgariSf 184
dimorphism, 98
Sea-anemone,' i op., ,313
Sea-campion, 198
Sea-cows, 490
Sea-elephants, 4S4
Sea-lavender, 276
Sea-lions, 490
Sea-squirt, 558
Sea-trout, 178
Sea-urchin, 32, 514, 549
Seals, 490
Secaky 3390.

Seiums novcboracensis, 355
Senedoy 177, 232, 355
Senedo campestris, 447
Sepia offidnalisy 416

INBBX— ORGANISMS

635

Serin fmch, 3 CmS, 444
Setinus cmarius^ 306, 444
Service-tree, 198
Shearwater, 286
Sheep, 82, 18$, 275, 541
hornless, 76
'isolation in, 186
Shelduck, 239
Shrews, 281

Shrike[s], 182, 193, 225, 236, 415
hybridization in, 252
Sicklebills, 183, 324
Sikne, 274

Sikne dliata, 145, 336

Sikne maritima, 198, 266, 268

Sikne vulgaris^ 266, 268

Silkworm, 53

Sirenia, 565

SiteXf 429

Sitta caesia, 219

Situtunga antelope, 236

Skua, 103

Skunks, 548

Skylark, 306, 426

Sloths, 490

Snail[s], 291, 431

Snake, 540

Snake, garter, 291

Snapdragon, 53

Snipe, 240, 516

Solanum demissum^ 198

Sohnum dulcmarar 277

Solanum mllis-mexkU 336

Soknohia, 141

Sorbus, 198

Sorex arrneus, 281

Sorex minutus, 2$i

Sorghum purpure<Hsericeum, 339

Soriddae, 281

Soroserist 377

Sparrow, 256, 326 n., 378, 448

fox, 182, 193, 196, 217, 220, 225,
23H

house, 94 n„ 256, 279, 448, 519, 521
Lincoln, 238
song, 219, 238, 272
tree, 279

Sparrow-hawk, 106, 282

Spartina alternijiora^ 341

Spartina tomtsendii, 146 * 34i» 3^4* 451

Speedwell, 269

Spider, red, 300

Spilosoma mendica^ 106

Sponges, number of species, 168

Sprosser, 246, 254, 288
Squirrel. 103, 184, 292, 193, 227, 488
red, dimorphism, 98
Starling, 18711., 424
Stegocephalia, 505
Stegosaur, 485
Stellaria media, $1^

Sterna, 279

Stick-insects, 102, 303 n., 459
Stoat, 281
Stock, 189
Stockdove, 310
Stonechat, 290
Stmthio, 476
Stumella, 251
Sturnus vulgaris, n.

Swallow, 279, 311, 424

Swallowtail, see Butterfly, swaEowtail

Sylvia, 176

Sylvia atricapilla, 306

Sylvia communis, 306

Sylvia curruca, *270, 306

Sylviinae, 307

Synagris cornuta, 321

Synentognathi, 158

Synodontis hatensoda, 415

Syrphus, 323

Taehyetes, 2%s
Tadoma tadorna, 239
Tamandua, 537
Tapeworm, 558

Taraxacum, 120, 137, 147, 166, 375, 477
Tarweeds, 276
Teal, 238
Teleosts, 544

Termites, 312, 482, 491, 554
Terns, 279

Tetrmychus opuntiae, 300
Thamnohia, 242
Thamnophis ordinoides, 291
Thera Juniperata, 185
Thrasher, 421 , 452
Thrush, song, 290, 306
water, 355
Thrushes, 290
Thyme, 198
Thymus serpyllum, 198
Thysanoptera, sex-determination in,

149

Tit, 176

cole, 218, 269, 280, 290, 309
crested, 269, 270

636 evolution: THE MODERN SYNXIUISIS

Tit (continued)
great, 243, 309
ioiig-tailcd, 247, 254, 280
marsh, 270, 289, 290
wiiiow, 270, 280, 289, 290
Titanothcrcs, 172, 488. 491, 495, 507,

508,515,534

Titaiiotheria, 507
Toad, firc-bcMicd, 246
Toads, 253
Tobacco, 1 1 8, 35011.

prcadaptatioii in, 52
Tomatoes, 337
Totmus hypokucus, 3 1 1
Toxostoma^ 421, 452
Tradescantici, 338, 375
Traqclaphtts spekii, 236
Tragulus, 183

Tree-creeper, 245, 284, 306
-cricket, 298
-frog, 282, 292
-heath, 232
Trkho^ramma^ Zi\

TrichoniscuSy 314, 336
Trichosurustuulpccuky 104, 203
Tridacnay 429
Trilobites, 508, 560
Trimtropis, 253
Tringa totanusy 403
Triticum speltOy 332
Triticum timopheeni, 345
Triticum vulgarcy 145, 332
TroglodyteSy 181, 223
Troglodytes musculusy 225
Trcglodytcs troglodytcSy 212
Troglodytes U hirtensis, 160, 176, 309
Troglodytes t. zctlandicuSy 176, 309
'Progons, 426
Trout, 178, 31^

Truncatinelia hritannkoy 196
TruncatineHa rmermiOy 197
Trypoxyiott, 321
Tulipa, 143, 378
spcciation in, 378
Tulips, 336, 519
Turbinella pirumy 177
TurduSy 290
Turdus erketorutHy 30 <>

Turdus mentky 266, 290, 306, 309
Turdus migratoriuSy 290, 310
Turdus philomcluSy 290
Turdus torquotuSy 266, 290
TUrriteliteSy 508
Twite, 266

Tykndms dipsaciy 300
Tympanuchus cupidoy 201, 426
Typothercs, 491

Ikay 541
UlmuSy 227
Ungulates, 490
Uredincac, 301
Uria aalgCy £05, 16 1

Venusia umkuktay 415
Vermworoy 251, 254
Veronkay 280, 393
Veronica hyhriday 269
Veronica spicaiay 269
Vertebrates, number of species, 168
Vetch, kidney, 356, 441
108, 109, 353

Viola kitaibelianay 336, 349
Viola tricolor y 5 1 7
Viper’s bugloss, 107
Viruses, 131
Viscum alhumy soH
Vitis lahruscay 3 14
Vole, 53, 105, 1 18
Vulpes, III
Vulpesfulvay 103, 185
Vulpes uulpeSy

Wagtail, 176, 179
Walnut, 437

Warbler, grasshopper, 306
sedge, 3(K>, 534
wiiiow, 278, 289, 309, 424
wood, 289

Warblers, 180, 245, 251, 307
Wasp, chalcid, 299
Water-boatman, 46H
-flea, n8
-snail, 236
Waxbell, 278
Wax-moth, 303, 459
Weasels, 255, 281
Weeds, 278
Whales, 240, 489
whalebone, 493
Wheat, 30H, 345
einkorn, 345
Whimbrel, 28 1
Whiiichat, 290, 307
White Admiral, 253

INDEX— ORGAHISMS.

White-eye, I 79 » 255, 272
Whitefish, 178
■Whitethroat, '3o6, 307
lesser, 270, 306

Waiows, 302, 345 » 351 , 353, 455
Wolf, 1 1 5, 307, 540
marsiipiai, 492
■Wood-ancmonc, 477
Woodlouse, 314 .

Wood-mouse, 225
Woodpecker, 223, 424
coloration in, 518
downy, 280

greater spotted, 280, 445
;,:.ha'iry, 280 ; ,
lesser spotted, 280
Wood-wasp, 429 :

Worms, number of species, 168
Wren, 212, 223, 225, 309, 424
Fair Isle, 202
St. Kllda, 160, .X7«5, 309
Shetland, 176, 309

Wryneck, 413

Xenarthra, 490
Xemtis megdotis, 442
Xiphophoms, 66, loi

Yeast, 121 '

Yellowhammer, 306, 309

Zebra, 217, 547
Zeughdon^ 505
Zoarces vivipams^ 223
Zonotrichia, speciation in, 378
Zmotrichia mpensis, 326 ti.
Zosierops, 179, 200, 25 s
Zosterops semgaknsis, 272
Zosterops virm$, 272
Zygoma JUipmdula, 293

AUTHORITIES

Adams, 431
Aldrich and Nutt, 214
Alexander, 443

Allan, 147, 167, 258, 352. 353, 355 . 4o8
Allec, 479

Allen, 211, 213, 250, 394
Alpatov, 220

Akenburg and Muller, 512
Anderson, 224, 338
Anon*, 520
ArkcU, 40<>, 5o<>

Arkcll and Moy~ 7 'homas, 409

Babcock, 344

Babcock and Stcbhiiis, 353, 372
Badenhuizen, 337
Baily, 236, 308
Baker, Stuart, 224, 252, 45 1
Bakcwcll, 1 86

Baldwin, 17, 114, 296, 304, 523

Ball, 431

Banks, 253, 264

Baiita, 52, 85, 118

Baiica and Wood, 52

Barber, 381

Barbour and Shreve, 234
Barrctt-Hamilton and Hinton, 271
Barrows, 76
Barton-Wright, 437
Bates, G. L., 193, 214
Bates, H.W., 465, 497
Bates, R. W., Riddle, and Labr, 450
Bateson, 23, 24, 28, 167, 169, 198, 250,
251,252, 520
Bateson and Bateson, 160
Bather, 508
Baur, 53, 64, 115
Beadle and i kagg, 279
Beauchamp and Ullyett, 121
Beebe, 218, 224, 3iO
dc 15 a r, 127, 527, 532, S 38 . S 55
Benedict, 55

Benedict (F. (k), Landauer, and Fox,

Benson, 272
Bent, 281
Bcquacrl, 321

Berg, L. S., 328, 497 11.

Berg, R,L., 71, 83. 113
Bcrgmafin, 211,212
Bergson, 28, 457, 475
Biddle, 72
Bird and Bird, 221
Blair, A. P., 253, 292
Blair, R. H., and Tucker, 425
Blakcsicc, 89, 330

Blakcsiec, Bergner and Avery, 198,

Blakcsicc and Fox, 53
Bbker, 421
Boik, 526, 555
Bonncvic, 62
Bonnier, 441
Borgstroin, 109
Bowater, 72 n.

Brewer, 573
Bridges, 360
Brierley, 69
Broom, 568 n.

Brough, 515

Bruno, 542

Buchner, 429

Buck, 236

Bumpus, 448

Burkitt, 36

Butler, 287, 544

Butler, Samuel, 458

Buxton, 305, 337, 422, 433

Buzzati-'Fraversc), 203

Caesalpinus, 263

Caiman, 399

(limeron, 425

Carothers, 253

Carpenter, 217, 336, 414

Carpenter and Ford, 416, 464, 515

Castle, 82, 525, 532

Castle and Nachtsheim, 511

Castle ancl Piiicus, 65, 70

Cx‘snola, 448

C:hampy, 536, 541

Chapman, A., 236

Chapman, F. M., 158, 160, 163, iHo,
199, 232, 242, 351, 270, 326 m, 378

INBIX— AUTHOIITIES

Chapman and Griscom, i8i,, 223, 225'

Gheesman and Hinton, 283

Chevais, 177, 536

Childe, Gordon, 566

ChristolF, 347

Christy, C., 192, 214

■Christy, Miller, 274

Clancy, 403

Clark, W. E. Le Gros, 396, 515
Clausen, 349

Clausen, Keck and Hiesey, 276, 43';;
441

Cieiand, 91
Clements, 441
Coleman, 467
Collins, 63

Collins, Holiingshead and Avery, 350
Colman, 222
Gomes, 415
Correns, 24

Cott, 102, 414 C, 432, 464, 468
Cowan, 392
Crampton, 232, 233
Crane and Thomas, 334, 352
Crew, 459
Crew and Lamy, 70
Crew and Mirskaia, 71
Crosby, 222, 313
Cross, 103
Cuvier, 391

Dale, 212, 423
Danis, 215, 223
Danser, 163
Darling, 103, 480
Darlington, 48,67, 84, 88, 126, 128, 1 30,
131, 132, 133. 134. I35» 136, 137,
140, 141 n., 144, 145, 150, 152,
328, 330, 331, 334» 335, 33^» 340,
34X, 350, 352, 3<52, 378, 382,477
Darlington and Upcott, 329, 339 n.
Darwin, 15, 23, 27, 30, 35, 55, ^ 07 ,
125, 127, 128, 153, 186, 204, 387,
390, 412, 416, 430, 533, 555
Davenport, 52, 85
Dementiev, 223, 245, 252
Desselberger, 551
Dewar and Finn, 189, 242
Dice, 182, 189, 225, 291, 294, 432
Dice and Blossom, 36, 188
Dickson, 472

Diver, 99, 167, 202, 291, 292, 322,
443, 520

Dobzhansky, 54, 55, 58, 60, 80, 86, 91,
92, 93, 115, ii7. ^31, 3:62, 163,
167, 194, 202, 214, 220, 262, 293,
323, 328, 338, 344, 345* 357, 358,
359, 406, 410, 533

Dobzhansky and Tan, 91, 93, 33®, ^64

DoHo, 501

Donisthorpe, 413 m

Donovan, 253

Doob, 557

Dowdeswell, Fisher and Ford, 410

Dubinin, 75, 117, 118, 363

Dubois, 539

'Duke, 300

Dunbar, 255

Dunn and Landauer, 76

Darken, 459

Edinger, 506
Eigenmann, 453, 454
Eimer, 394
Eker, 64
Ellers, 176
EIofF, 314

Elton, 15, 103, no ff., 169, 187, 209 11.

Eltrii^ham, 191, 217

Emerson, 312, 419, 482 n., 491, 554

Endrodi, 176

Engels, 421, 452

Eslick, 320

Evang, 70

Evans and Vevers, 195
Everett, 578

Faberg4 337

Fantham, Porter and Richardson, 300
Federiey, 76, 348
Femald, 376
Finn, 315
Fiscber-Piettc, 319
Fisher,}., 202, 217, 269, 310, 44S
Fisher, }., and Waterston, 445, 480
Fisher, R. A., 21, 26, 27, 28, 33, 55,
56, 57, 58 n., 72, 75, 77, 79, 81,
83, 94, 97, 99, ^^3, ^24. 127,

128, 129, 152, 204, 260, 463, 4^5,
473, 474, 509, 517, 53i
Fisher and Ford, 33, 127
Fisher* Ford and Huxley, 53
Fitch, 214, 291

Fleming and Snyder, 219
Forbes, 244

640 evolution: THB MODERN SYNTHESIS

Ford, E. B., 58 !!., 62 n,, 70, 80,
82, 83, 85, 94* 95» 9<5, 100, loi,
103, 105, 106, 112, 123, I24» 217,
222, 293, 393. 470. 513. 5X<5. 5x8,
521, 531 II., 533 . 552
Ford and Ford, II2

Fordand Hii)clcy,74, 527, 528, 531, 533

Fornsosov, 1 14, 279

Fox and Wingfield, 273

Fox, I ). L., and Pantin, 100

Fox, H. M., 273, 435

Frankcl, 393

Fries, 177

Oalton, 24, 151, 152
Cjirncr and Allard, 52
Garrett, 233
Garstang, 532, 562 n.

Gates, I2T, 199, 354 n.

Gausc, 121, 279, 548
Gausc and Smarageiova, 315
Clcrard, 263
Gerould, 98, 117
Gershenson, 481
Gesner, 263
Ghigi, 224
Gidlcy, 487

Gilniour, 342, 390, 399, 401
Gihnour and CJrcgor, 203
(ihdkov, 218
Glass, 121

Glogcr, 21J, 213, 393
Goethe, 391

Goldschmidt, 53, 74, 117, 127, 128,
162, 1 91, 197, 202, 21 1, 215, 216,
220, 434. 43<5, 45S, 456, 497. 5o6,
51211,, 527, 528, 529. 531, 533.
543. 552. 553

Cioldschinidt and Fischer, 98
C.oiizalcz, 69
Coodspeed, 344
(iordon, C., 75, 117
Gordon, C. and F., 545
Gordon, C-, and Sang, 63
(Jordon, M., 66, 71. i(X>

Gowen and Gay, 138
Graves, 534

CJrcgor, 177, 223, 275, 276, 277
Gregory, 487, 501, 502, 55X
(Jriiincll, 176, 188, 189, 192, 226
Gross, 103, 201
GriinelxTg, 49, 62, 63
Culick, A.. 50, 183, 325, 327

Gulick, J. T,, 232, 234, 394
Gustafsson, 93
Guyer, 45H

Hackett, 317

Hackett and Missiroli, 317
Haeckcr, 546, 547, 553
Hagcdooni and Hagedoorn, 202

Hagerup, 337

Hakluyt, 263

Ffeldanc, 15, 21, 27, 28, 30, 33, 34, 53,
55,56,58,79,82,100,112,117,119,
123, 127, 128, 129, 189, 190, 219,
337. 361, 396, 463, 475. 4B2, 4B4.
494 506, 508, 509, 51X. 5?'5.
526. 527, 531. 555. 565, 570. 571
Hall, A. a, 37B
Hall, E. R., 217, 255
Hamilton, 234
Hammond, 541, 542
Hardy, 94 11.

Harland, 54, 70, 77, 78, 79, 346

Harland and Attcck, 79

Harms, 553

Harrison, H. S., 534

Harrison, J.W. H., 94, 95, J20, 195, 458

Harrison, W. H., 302, 304

Harrison and Carter, 253

Hartert. 176, 239, 245, 248 n., 249

Hasbrouck, 106

Hasebroek, 94

Hawkins, 32, 38, 172, 5:67

Hays, 59

Heinroth, 306, 425

Heinroth and Heinroth, 290, 310

Herbst, 458

Herrick, 310

Hersh, 535

Herzberg and Masslcr, 536

Hesse, 168, 1 79, 2 1 3 , 4 1 9

Hesse, Alice, and Schinidt, 129, 430, 486

flcwitson, 159

Hildebrand, 107

Hiie, 178

Hill, A. W.,308

Hill, J. E„ 462

Hill, Osman, 428

Hinton, 191, 256, 258, 271, 281

Hogben, 20, 27, 285, 313, 475

Holmes, 416

Horuell, 177, 520

Hough, 472

Hovanitz, 262, 463

INDEX — ^authorities

Howard, 308

Hubbs, 177, 195, 302 , 223, 348, 454,

54 ^

Hubbs ond Cooper, 442
Hubbs ud Hubbs, 146 n., 316
Hubbs and Trautman, 282
Hughes, McKenny, 94, 458
Huskim, 102, 332, 341
Hutchinson, 73
Hutchinson and Ghose, 77
Hutt, 450
Huxley, A., 573
Huxley [J. S.], 13 n., 35, 45, 66. 97. 103,
127, 128, 129, 130, 170, 192, 202,
206, 210, 212, 218, 226, 288, 289,
334, 354, 435. 436, 455, 484, 504,
538, 529. 530, 534, 535, 536. 537,
538, 541, 543, 543, 547, 555.
550 n., 56911., 572, 57711.

Huxley and de Beer, 547, 553 n.

Huxley and Carr-Saunders, 458
Huxley and Ford, 529
Huxley and Haddon, 354, 399
Huxley and Teissier, 529
Huxley, T. H., 391
Hyatt and Wurtemberger, 508

Iljin, 64, 1 1 5, 307, 546
lijin and Iljin, 546
lijina, 185
Ingoldby, 192

Jameson, 187 n.
Janaki-Ammal, 339 n.
Jenkin, Hecming, 204
Jenkins, 372
Jennings, 84, 328
Johannsen, 52, 118
Johnson, R. H., 550
Johnson and Newton, 307
Jollos, 509 11.

Jordan, D. S., 186
Jouard, 248
Jourdain, io2, 256, 31 1

Kalabuchov, 436
Kalmus, 433, 451
Kammerer, 232, 458
Kappers, 538
Karpcchenko, 347
Keith aiid McCown, 354

Kennedy, 194
Kerner, 416
Kerner and Oliver, 107, 108
Kikkawa, 364
Kimey, 163 n., 348
Kirikov, 103
Klauber, 537

Kleinschmidt, 163 n., 248 n.. 394

Knight, Thomas, 264

Korotneff, 181, 496

Kosswig, 66

KostofF, 343

Kovalevsky, 22, 395

Kramer and Mertens, 300, 23 ^83

Krogh, 57

Knimbiegel, 206, 314.

Kuhn. 68

641

Lack, 36, 254, 289, 310, 3 1 1, 326, 327,

35<S, 519

Lack and Venables, 269
Lameere, 536
Lammarts, 35on.

Lamoreux and Hutt, 450
Lamprecht, 53, 115, 343
Landauer, 76, 119
Landauer and Dunn, 119
Landauer and Upham, 119
Lang, A., 99, 529
Lang,W. D.,409, 515
Langlet, 224
Lankester, Ray, 458
Larambergue, dc, 313
Lawrence and Price, 64
Lea, 50
Lebedefii 76
Ledingham, 343
Leitch, 410
Lepekhin, 103
Levick, 518
Lewis, 140

UHeritier, Necfs, and Teissier, 120, 453
L’Heritier and Teissier, 3 58
Linnaeus, 168, 264, 387, 390
Linsdale, 182, 193, 432
Lloyd, 58, 202

Lloyd Morgan, 17, 114, 296, 304, 523

Lockley, 195

Loeb, 544

Longley, 169

Lomiberg, 538

L^ppenthin, 219, 226

Lorenz, 45, 289

642 B¥OtUTION: THE MOBEIN SYNTHISIS

Lowe, 94 a., 1S3, 203, 242, 356

Lowe and Macfcwordi-Pracd, 190

Luhring, 99

LuE, 501

Lamer, 539

Lynes, 176

McAtee, 414
MacBride, 37, 39
McCabe and Miller, 355
McConaill and Ralphs, 527
MacDougal, 442
McEwen, 52
Macfadyen, 408
Mackenzie and Muller, 92
McMeekan, 542
Malinovsky, 332
Manton, 337
Manwell, 319
Mardalewski, 82
Marett, 567 n.

Marsden-Jones, 277
Marsden-Jones and Turrill, 258, 268,
3S6,44I»5I9
Marshall, F. H. A., 442
Marshall, W. W., and Muller, 70
Mather, 67, 87, 106, 107, 108, 115,
140.313,477,545
Mather and de Winton, 107
Mather and Dobzhansky, 333
Mather and North, 76
Matthew, 32, 377, 39<5, 488, 498
Matthews, 183
Matthews, L. H., 545
Mayr, 160, 163 n., 167, 169. 184, I99,
202, 223, 240, 255, 372, 273, 284,
289, 305, 403

Mayr and Greenway, 215

Mayr and Rand, 103

Mayr and Ripley, 199

Mayr and Serventy, 214, 516

Meinertzhagen, 187 n., 192, 255 n.,

311.433,445, 462

Meise, 195, 248, 249, 250 11., 256

Meisenheimer, 545, 555

Mellanby, 435

Melville, 227

Mendel, 390

Metalnikov, 459

Metcalf, 393

Metz, 148

Meyer, 315

Miller, A. H., 182, 193, 224, 236, 248,
251,421

Miller, G. S., 183, 240, 487

Mller and McCabe, 238
Mobius and Heinke, 431
Mohr, 71 n., 80
Moline, 458
Moiony, 289
Moore, A. R., 527
Moore, J. A., 435, 444
Mordvilko, 183, 324
Moreau, 192, 194
Morgan, Lloyd, see Lloyd Morgan
Morgan, T. H.. 23, 26, 63. 69, 4I7,
418,475.498,528

Morgan, Bri<%es and Sturtevant, 510.
511, 512

Morgan, Schultz and Curry, 86
Morrison, 264

Moy-Thomas, 409, 542, 543, 545
Muller, 27, 48, 49^2.-* 50. 5X, 58, 68,
83,88,89,92,93,127,138,143, 148,
185, 255, 259. 333, 357, 358, 359,
360, 361, 3<52, 365, 366, 370, 455,
476,502,573

Muller and Pontecorvo, 359
Muller, Prokofyeva and Raffel, 92
Miintzing, 88j 222, 33611., 337, 343:,
347,375

Murphy, 285, 326 n.

Murphy and Chapin, 194, 327

Nabours, 99
Nash, 439

Needham, 502, 562, 565 n.

Needham, Huxley and Lerner, 529 n-
Needham and Lemer, 529 n.
Nicholson, A. J., 444, 467, 483
Nicholson, E. M., and Fisher, 279
Nilsson, 345, 353
Nobel, 282

Noble and Bradley, 35
Noble and ¥ogt, 288
Nopcsa, 505
Norman, 158, 415, 45<5

Oken, 391
Oldham, 270
Omer-Cooper, 247
Orr, 240

Osborn, H. F.,. 304, 486, 487, 488, 515,
534

Owen, 391

Paley, 23, 467

INDEX— AUTHORITIES

643

Palmgren, 234
Panikkar, 455
Parker, 285
Parkin, '429

Patterson and Grow, 364, 368

Patterson and Muller, 50

Patterson, Stone and GriHen, 367

Panlian, 97

Payne, 459

Pearson,}., 104

Pearson, Karl, 24, 151

Perlova, 336

Pfliiger and Smith, 213'

Phillips, J. C., 531
Philips, W. W.A.,415
Piaet, 501
Piisbry, 232, 234
Plate, 2d6n., 383 n., 500, 523
Plough, 85

Plough and Ives, 509 n.

Plunkett, 73, 83
Popham, 468
Porter. 213, 235

Poulton, 30, 416, 458, 498
Pratt, 283

Promptoff, 183, 308
Pryor, 451
Przibram, 213-

Pumphrey and Rawdon-Smith, 298
Pumphrey and Young, 429
Punnett, 102
Punnett and Bailey, 53
Punnett and Pease, 72

Quayie, 471

Radi, 22

Raffel and Muller, 49 n.

Raffles, 264
Ramsbottom, 300
Ranger, 428
Ray, 264
Reeve, 537

Reeve and Murray, 53d

Regan, 157, 160, 178, 181, 223, 503

Reich, 305

Reinig, 130, 225, 226

Renner, 91

Rensch, 154» 163, 174. I75* I79, 180,
211, 212, 213, 214, 215, 216, 223,
234 and n., 239, 243, 244, 245,

405,543

Rhine, 574
Richards, 321, 410
Richter, 244
Riddle, 453
Ritchie, 183
Robb, 536
Roberts, A., 214
Roberts, J. A, F., 82
Roberts, Morley, 563 n., 574
Roberts and White, 82
Robson, 30, 153, 313
Robson and Richards, 30, 39, 153, 224
320, 449 n.

Rothschild and Hartert, 215, 21 1
Rothschild and Jordan, 21 1
Rowan, iii
Rubtzov, 99, 516
Russel, 428

Ruxton and Schwarz, 253

Salaman, 198

Salisbury, 269, 273, 274, 276, 278,
438, 44d, 452, 4d8, 554
Salomonsen, 195, 212, 262, 551
Sander, 345
Sanderson, 313
Sarasin, 394
Saunders, 26, 189
Schaeffler, 240

Schilder and Schiider, 177, 179 n.
Schmidt, 223
Schnakenbeck, 177
Schultz, 48 n., 528

Schultz, Caspersson and Aquilonius,
138

Schwanwitsch, 550
Schwarz, 247
Schweppenburg, von, 244
Scott, A. C., 383
Scott, J. P., 502, 550
Scott, W, E., 305
Seth-Smith, 433
Sexton, 314

Sexton and Clark, 75, 511
Sexton, Clark and Spooner, 70
Shaw, Bernard, 458
Shortridge, 218
Shul, A. F., 26, 414
Shul, G.H., 34d
Sick, 422
Sikka, 350

Slow, ds, 79, JIS, ii6, 228, 457, 533
Sinnott, 528

644 EYOIUTION: THB MOBEIN SYNTHESIS

Sinnott and Dunn, 62
Sinskaja, 275
Slack, 370 ■

Sladden and Hewer, 303 n., 459
Smart, 169
SmitE, Eliot, 570
Smith, Geof&ey, 536
Smith, Malcolm, 503
Smith, S. G., 3x4
Snyder, J. O., 17S
Snyder, L. L., 219
Sokolov and Dubinin, 330
South, 445
Southern, 105, 161
Spath, 508

Spence and Yerkes, 526 n*, 570
Spencer, 53, 61, 119, 291, 357 » 358»
367»4I0

Spencer, Herbert, 565 n.

Spooner, 315
Stadler, 306

Stanford and Mayr, 272
Stapledon, 121, 276

Stebbins, 353, 372, 377 |

Stegmann, 244 1

Stensio, 31 ;

Stephenson, 313

Stem, 328, 330, 332, 333

Stirton, 488, 498

Stockard, 71, 506, 553

Stone and Griffen, 366

Stonor, 240, 425, 426

Storey, 312

Stovin, 195

Stresemann, 160, 163, 179, 184, 200,
237, 242, 247, 248
Strohl and Kohler, 450
Stubbe and Vogt, 369, 51 1
“Student”, 116
Stull, 540

Sturtevant, 27, 77, 137. 357 » 397 » 534
Sturtevant and Beadle, 363
Sturtevant and Dobzhansky, 365
Siiffert, 415
Sukatschew, 120

Sumner, 96 n., 187, 188, 189, 190, 208,
210, 216, 291, 434, 479
Suomalainen, 106
Sviridenko, 271 n.

Swarth, 44, 182, 183, 196, 217, 225,
242,326

Sweadner, 219, 253
Swellengrcbel and de Buck, 317, 318

Swinnertoh, 409, 488, 489, 515 n.,
528, 53 i» 53 < 5 » 537

Tansley, 120

Taverner, 251, 252, 354 , 355
Tchemavin, 315
Tedin, 115
Thayer, 36

Thomas and Wroughton, I 93 » 219,
227

Thompson, D. H., 241
Thompson, D*Arcy, 494 n., 542
Thompson, W. R., 428
Thomsen and Lemcke, 94, 458
Thomson, A. L., 238
Thomson, G. H., 573
Thorpe, 295 c, 299, 311, 312*

319,428,459

Thorpe and Jones, 3 02
Ticehurst, 176, 245

Timofeeff^Ressovsky, 50, 73 n., 103,
120, 126, 191, I99» 203, 210, 238,
261, 410, 436, 444

Timofeeff-Ressovsky, N. W, and
E. A., 358
Tiniakov, 51
Tischler, 337, 340, 347
Trueman, 489
Tschermak, 24

Turesson, 200, 275, 277, 375, 437
Turril, no, 147, I 57 » 166, 171, I 79 »
180, 197, 258, 267, 273» 277, 352,
353 » 37 S» 388 m, 390, 393 » 398 ,
401, 405, 406, 519 ^

Tyrrel, 574

von Ubisch, 549
Upcott, 143
Uphof, 107
Uvarov, 202, 431

Vandel, 88, 314, 336
Van Oordt, 310
Vavilov, 129, 408
Velikokhatko, 273
Vesey-Fitzgerald, 480
Vilmorin, 16
Vogt, 214

Vogt and Vogt, 550

de Vries, 23, 24, 38, 91, 150, 204, 330

INDEX— AUTHOBIXIES

Waagen, 174

Waddington, 63111, 131, 148, 345
457, 555

Wallace, A. R., 30
Walther, 94
Walsh, 299

Wamke and Blakeslee, 142
Warren, 176, 183
Watson, D, M. S,, 192, 504, 544
Webber, 350 n.

Weidenreich, 555, 570
Weismann, 17, 23, 28, 29, 30, 418
Weiss, 71
Welch, 234
Weldon, 24, 1 5 1, 448
Wellington, 314

Wells, Htodey and Wells, 61 n.,
55dn., 559^-. 570
Went, 428
Westell, 544
Wheeler, 467, 495
White, E. L,409
White, Gilbert, 289
Wliite, M. J. D., 48, 142, 149, 334,

348,357

White, O. E., 437, 455
Whitney, 435

Willis, 34, 169, 197, 204, 205
Wilmott, 198
Wilson, 170

645

Wir®e,99,i44
Wingfield, 435
Winter, 116

Winterbottom, 525 n., 545,' 555
Witherby, 176, 218, 221, 309, 444, 445
Witschi, 235, 419
Wolf, 328

Wood, H. E., 498, 506
Wood,T.B.,76
Wood-Jones, 286, 422, 427
Worthington, 178, 181, 324
Wright, Sewall, 21, 27, 30, 33, 57, 58,
59, 60, 82, 83, 113, 126, 127, 128,
155, 186, 194, 197» I99» 200, 208,
229, 232, 237, 242, 259, 260, 265,
326, 3d2, 368, 474> 476, 479.
501, 502, 550

Yocom and Huestis, 188
Yonge, 181, 429, 430, 492
Yule and Wilb, 205

Zarapkin, 176, 548

Zarapkin and TimoSefF-Ressovsky
{H.A.), 549

Zimmermann, 105, 118, 271
Zuckerman, 403, 519
Zuitin, 55, 358

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